I can speak only for iPoker (Mansion).
The traffic at $1/$2 FLHE is 3rd best you'll find behind Stars and FTP. It is also tighter than FTP and (by a good source) waaay tighter than Pokerstars. You'll have trouble finding good tables a lot of the time due to a high number of TAG regs playing there - and I mean a lot. Four TAG regs one loose/passive and you is generally a good table. It does appear to loosen slightly at $2/$4 oddly enough.
I've never had any problems with their support or the site itself. The iPoker software isn't the prettiest in the world, but I played on Mansion/Titan for 14 months and never had any troublesome issues. Their VIP programs are pretty good, but then again so is the rake - I have 3.2BB/100 in paid rake at $.50/$1 over 43k hands compared with 2.7BB/100 over ~10k hands at FTP. You can counter some of the impact of this by playing during four hours of happy hour on Saturday/Sunday 16:00 - 18:00 GMT - but you are ofc fighting every reg then for the privillege!
A further note on the software, iPoker did release a new software update about 7 weeks ago which apparently can make multitabling more than 4 or 5 tables difficult due to lagging - depends on how good your processor is I guess. I've not had any problems with that though. They have said they're working on reducing the load the software puts on your PC processor - though not sure what the status of this is.
Hope this helps.