I didn't try biking but did see this on wikitravel:
Bicycles are a very convenient option to visit Angkor Wat and Angkor Thom, the little circuit or even the big circuit - depending on time you have and how big fan of Khmer temples you are. Renting a bike in Siem Reap is easy and cheap (USD1 per day, in most of places you don't even have to leave passport, locks for bikes are provided, check the bike before and ask for some amendments if needed, eg, pumping air, oiling the chain). It is about 6km from the city to Angkor Wat (if you go first time, make sure you go by the Visitors Centre which is the only place where you can buy passes). In the little circuit most places are at most 15 minutes away from each other by bike, so it is actually not a problem for a regular tourist (without much biking experience) to visit Angkor Wat, Angkor Thom and spots on the little circuit in one day. If you are willing to get up early and start your trip 06:00 (it is not uncommon to see a bike rentals open from 04:30) it won't be a problem to visit all above plus the big circuit (where spots are 30 minutes away each other by bike) in one day. Take into account your shape and visiting preferences. If you bike a lot at home - you can easily get around much quicker. If you enjoy Khmer architecture more than the regular Angkor visitor, it is recommended you reserve at least 3 days for the trip (and it doesn't matter if you go by tuk-tuk or by bike). It is a good idea to take a lot of water with you (rent a bike with a basket!), but not a big problem if you run out of it during your trip. Around every temple in Angkor park you can buy some food and drinks (it's just more expensive than in the city, around US$2 per big bottle of water in the Park). Cycling in Angkor Park is safe (traffic is low), pleasant (nice views and a lot of trees providing shadows in sunny days) and, last but not least, it saves you a lot of hassle of dealing with tuk-tuk drivers.