Azureus does not have a very good speed limiting mechanism, and previous poster is correct that upload can stop from browsing as well, if you want to limit speed you should try something like netlimiter (www.netlimiter.com). be warned though this software uses LSP (Layered Socket Protocol) and can conflict with your firewall software e-g ZoneAlarm....
i use ABC and have had no problems, sure thing slow down a bit if theres alot goin on but i just thought that normal. Bit of a 256u/l bottle neck as it were.