The RESO, Montreal's underground city-within-a-city, is very easy to navigate with well sigposted "streets". There are maps at the main entrances, computerised search facilities and even information desks.
For a downloadable map showing the RESO in relation to Downtown as a whole click HERE
Or for a schematic map showing the main buildings and Metro stations click HERE
Honestly, I believe walking was the best way to get around Montreal. I even walked home at night on St. Catherines street alone at night and had no trouble at all (though I was warned not to walk St. Catherines alone, I did.. and had no problem). Taxi drivers are nice (if you speak french, thankfully I had a french friend with me when I had a taxi) they try to rip tourists off so try and pretend where you know where you are going. My friend had to pay a $50 cab ride that should of been a ten minute walk. So try to avoid taxi or atleast know your way around Montreal so they don't rip you off. Or, try to speak french with a taxi driver they will respect you more as I noticed.
I find it very pleasant to walk around downtown Montreal. The atmosphere is upbeat yet not claustophobic, livefull yet not suffocating. Sometimes, when alone at my place in the middle of the night, I decide to pick up my coat, cross the Jacques-Cartier bridge and walk for hours around St-Denis, the Moutain, le Plateau....
If you are not taking the Metro and the weather is agreeable, walking is a very nice way of getting around Montreal. This is true especially if you are downtown or in the Old City where streets are close together and there are lots of interesting things to see. Walking, as opposed to driving, will give you the opportunity to really notice and appreciate your surroundings, the architecture of buildings, locals, cute shops, cafes, etc. The exercise is also wonderful if you've had a few good meals!
I think walking is a great way to see Montreal. There are so many interesting things to see that you would miss if you were on a bus or the subway. Old Montreal and the port area is an especially good area to stroll around and see all the great little shops and places there are to discover!
We rented a car and drove around. When we got to Montreal we parked it somewhere (plenty of space) and walked around. Most things are close together so it was great to walk around.