I like the info TC gives me, but it's a resources hog. Since I disabled CTRA it's my largest addon at load time, and anytime my gear changes (a lot as a druid) or my attack power changes (tons with buffs & procs as a fury warrior) it gobbles up a ton of mem & time recalcing how much abilities hit for.
I think the biggest thing about TC in its current form is that it calculates the AC for every mob/player you fight and stores it. I noticed this and turned off the option that factors mitigation into how hard you hit, but that only removes the calculations from the tooltips, it still saves the data & loads it from savedvariables.
I know that of all the non-ace2 mods I use, this one is probably most in need of some performance tuning.