I've never used any other clients besides ABC (basically I'm to lazy to change) and I have no reason to. At first I thought ABC sucked ass and was really slow, but after playing with it, adjusting settings etc.. I found out a way (for me) to get fast speeds relative to the popularity of the file. Maybe it will work for you.
Say you're dl'ding 3 files and there all popular (as in over 50 peers and 10 seeds). Let ABC use the automatic settings for about 15 min, even if the speeds don't start out that great. After that time, turn your local upload settings for each file to 10 or 15 and the uploads to 5-10 on each. With that you'll find out which files are getting the best speeds and which ones are not, so you can go back and adjust the settings accordingly to not drown in your upload speed vs download speed.
If all your files are speedy then don't change anything (obviously), but if 2 are slow and one is fast make sure the slower file upload speeds are set accordingly vs its download speed etc.. And each file should not go above 10 for uploading to help the slow one out. (unless it's a sure thing with your connection speed).
I just thought I'd post this because I was one of those people who always complained "bit torrents sucks!...it starts out so slow!..etc" If you really want it to get to work you gotta play with it and you'll be surprised on how fast it can be.
I'd like to hear other peoples tips as well