They can be a pain to bleed. My Gary Fisher HiFi has some Avid Elixr CR's. I really like them. Good braking performance and rather robust. The problem is when air does get in the system. All of a sudden you have a brake that goes out and a tree coming at you really fast. Not fun. I have not had it happen to me. My friend has some brakes (I think they are Avid Elixr 3's), but they really stink. His rear brake consistently gets air in it. I told him to upgrade to some higher end Avids, but he keeps trying to work out the problems.
I'll keep my hydraulic's on the Gary Fisher and my cable disc system on my Hardrock sport...they both have pro's and con's. I like the cable on the HR because it is really low maintenance and a very simplistic design. On thing to battle cable stretch is brake earlier and easier. Yanking really hard at the last minute will stretch any cable, disc or rim brakes. I brake hard on the hydraulic system since I know I can. On the cable system, I brake earlier and softer.
Oh, and this may help:
http://www.mtbr.com/cat/brakes/disc-...s_1507crx.aspx