If you need to change money in Tashkent, go to the bank early in the morning. Otherwise, they will not have any Soums (Cym) and you will have to resort to using store owners and taxis and they will give you a less favorable exchange rate. Also, if you travel on a tourist visa, you may need the bank receipts to account for the money that you declared when you entered the country.
Changing money in Uzbekistan is still a hassle. While strict controls that created two markets is pretty much gone (the rates at the bank are very close to small changers), problems remain.
I understand that there still are some controls and this in turn makes it difficult to get cash when changing money -- they just run out. Whatever the reason is behind the trouble, be prepared to only change $50 rather than $100 and make sure you change enough in Tashkent because it is even more difficult outside the capital. Banks will tell you to get there 'early' before they run out of money. Early means before 10am.
There are 1000 som notes around, worth about $1. Most of the time you change money you'll be handed stacks of 500s and 200s, so changing $50 might not be such a bad idea!
The usual practice of needing crisp and clean bills applies -- anything less than perfect could get you a lower rate and they may flat out refuse to change it at any rate.
Particularly on the metro, be carefult to avoid corrupt police. While most say the problem is getting better, police will on occassion demand your passport and there are stories of finding 'problems' and demanding a fine be paid.
Sobir Rakhimov metro station near Hotel Tara is particularly bad. Just claim your passport is at your hotel or offer them a copy. Whatever you do, don't follow them into any room away from other people. On the plus side, I found that being somewhat uncooperative gained support from Uzbeks -- maybe they can't get away with acting that way.