Hi. I've done it.
The best route in to Lao from Ubon is of course Pakse.
From Pakse you can easily (by Lao and Khmer standards) organize the trip to Phnom Penh from a guest house.. or there's a bus company with a store front on the road parallel the Xe Don just north of the bridge that crosses it from Pakse.. they run occaisional trips south and even up and across the hills in to VietNam.
The bus operator picks you up in a van.. takes you down past Si Phon Don and down to the border (about 2-3 hrs south of Pakse)... From there.. you will have an antiquated bus (likely with no toilet) waiting for you on the other side.. Hop in.. and get ready for 10+ on that one.. The trip happens once a day and gets you in to Phnom Penh at night.. so tuk-tuk/ motorcycle taxi is usually your only bet to get in to the city proper and to your hotel.. and better to grab one quickly because where the bus drops you can be a dead location where they park the bus.. just a few "in the know" drivers waiting for you guys.. then, nothing.
The other option is to cross in through one of the border posts in NW Cambodia directly.. but it is a logistical nightmare.. a volitle border spot.. and the least developed part of the country.. making travel very "adventurous"..
If you don't like this super-bus option.. Ubon to Phnom Penh overland is still best done via Pakse and down along the river by any means.. Kratie is a nice stop over.. as is Si Phon Don, of course.. Strung Teng is a logistical opportunity but nothing more than a dirt strewn truck stop type of town.
Every single guest house and tour agency will offer bus tickets to anywhere you need to go. The problem is that you never know just how much extra you are paying for this service. Most places will mark up 10%, but others will mark up more than 50%.
KVT offers service between Pakse to/from:
Vientiane (Sleeper and seated bus)
Champasak (Ban Muang)
Don Khong (4000 Islands)
Don Det / Don Khone
It is best to shop around. However, the bus company itself does publish prices, so it would be recommended to consult this before purchasing anywhere. If you buy from guest houses or agencies, you may pay a little more, but they may also include the tuk-tuk transfer from your guest house to the terminal and things like that, so consider that in your calculations.
Prices for the sleeper buses are per person, based on a double-bunk. If you want a single bunk, you must pay ~30,000 kip more.
The sleeper buses do not appear to have assigned 'seats' on the tickets, nor was my experience that they were first come first served. Do what you can to avoid being in the back of the bus, directly above the engine. This bunk has reduced head height and is ungodly hot due to the heat of the engine. Regardless of where you seat, it is recommended to bring those personal eye shades since the windows will not likely have curtains.
Locals will likely be on the same bus as you. Be aware that it is not typically accepted to bunk with a local woman unless you are of the same family, so don't even attempt it. Try to get them to arrange bunks so that foreigners can be bunked together if you are not already traveling as a couple.
The old website was: http://www.buslaos.com/
But this may not be functioning any longer. The best schedule I found is in the link below.
Yes, almost all guesthouses and travel agencies will offer mini-van transportation to Don Khong, Don Det and Don Khone starting about 50,000 kip. Departures will be in the morning and bus will take about 2 hours.
However, if you enjoy traveling a bit more "local" style, you can always take a local bus. The travel time will be about 30 minutes longer, but its more authentic in my opinion and you have the potential and you will travel in an open-air side seated bus/truck. Price is 30,000 kip.
To get the local bus, you need to take a tuk-tuk to the "8km bus terminal" (pay no more than 15,000 kip).
If you are going to Don Khong, the bus will drop you off at the access road, from which point you must walk 0.8km to the river bank and take a (10,000 kip) ferry.
If you are going to Don Det or Don Khone, stay on the bus an additional ~30 minutes and similarly walk over to the river.
Took local bus from Pakse to Savannakhet.
Departure times from Pakse are approximately 7am ; 9am ; 1030am
Price is 35, 000 kip. Bus was about 4 hours.
Price does not include tuk-tuk to get to the local bus terminal. Should pay no more than 10,000 kip for the bus station.
The bus will stop at least once on the way and allow some hawkers on to sell some food/drink.
There are nearly daily flights at 6.30am and 7.30am. Easy booking at Lao Airlines' office in Vientiane, 2, Pangkham Road. I booked three days in advance, but there were still several empty seats on the plane. Fare was 1010000 Kip (120 USD) one way in Jan. 2009, the flight takes about one hour.
I've used both the overnight VIP Bus with reclining seats and more recently, last month, the VIP sleeper bus. The catch of the overnight sleeper bus is that the VIP bus company we used did NOT tell us that it slept 2 to a bunk. Makes for a somewhat interesting bunk assignment with a fair amount of switching around the bunks since some local women were travelling alone and it's a major no-no in Laos for young women to be sleeping with men they don't know. But other than arriving at Thakek at 1AM in the morning, the ride was quite comfortable and I was able to get a fairly decent night's sleep. the bunk was relatively clean but cramped for 2 people.
If you're travelling alone, then the recling VIP bus is probably the better option for a female. I've used it some years back and managed a decent sleep as well. But much depends on your sleep patterns and how adaptable you are.
Just a word of advice with regards to buying your bus ticket--it's best to shop around a bit because in Vientiane, quite a number of the guesthouses and hotels and travel agencies really overcharge for the bus fare, some overcharging as much as 50%.
There are 2 VIP buses leaving daily to Ubon from Pakxe VIP bus terminal. The fare at terminal is 60 000 KIP or 200 Baht per person, first bus leaving at 8.30 in the morning and last one at 4.00 pm. Note - if you take the last one you'll have to pay a fee to the Lao immigration officer which vary from person to person. I was told to pay 50 baht and when I presented 20 000 kip bill I didn't get any change back. Some other people got away by paying 10 000 kip, some paid nothing. The reason for this fee is... so they say... to pay for their extra hours of work, since they would officially stop working at 4 pm.
There are several agencies and guesthouses that rent motorcycles and usually they charge 80 000 kip for one day. That can be good alternative to tour buses if you want to explore larger area around Pakse by yourself, seeing waterfalls (Tad Fane or closer) and stop by the fruit stalls on the way to Paksong, Tad Fane etc... driving towards higher elevations of Bolavens plateau can mean also more unpredictable weather (rain, storms and other natural beauties of the sky) therefore bringing a rain coat or warm jacket even when the temperature in Pakse city is near 35 degrees, isn't bad idea at all. We tried Bolavens plateau twice and we were caught twice by heavy rain or thunder up there, once had to cancel our Tad Fane visit.
You can book your ticket to 4000 islands by any travel agency or your GH in Pakse. Minibuses with AC will cost you about 60 000 KIP one way and they can get well crowded with tourists in peak season. It takes about 2 hrs driving to Nasakan village and there you will have to take a boat to Don Khone island or more popular Don Dhet (backpacker island). Some agencies also include the boat price and therefore charging a bit more for the bus, but if you want to get a boat by yourself, add another 20 000 kip to Don Khone (cheaper to Don Dhet). The alternative to mini buses are local buses or pick-ups from the southern bus terminal. It's cheaper, but it takes longer time to get you there as they stop on the way by villages to up-download people and their things (and animals), more dusty and absolutely more crowded and less comfortable (no air con, no comfy seats)... though it's an experience to try a pick-up if your body allows.