i just made this exact transition. went from 7.4HD on tiger to 10.3HD on mountain lion, and i can tell you it's a giant leap.
up side: ADC bump to 16k rules, disk cache rules, and clip gain and real-time fades are next level shit.
down side: for me, being a creature of habit, having to get up to speed with the new layout and new features all at once has been challenging. luckily i ran HD 8 for awhile before going back to 7 (stability issues with 8 on our old PPC G5--just wasn't cutting it), so i had some time logged on 8 at my place (and 8,9,10, in other studios), which has definitely helped a bit in terms of getting used to the layout on my own rig.
STABILITY. in my room, now 10 days into the transition to HDX, we still can't seem to get the system totally stable. have had all kinds of trouble, which is disappointing. (smokin' new 12-core westmere, fresh SSDs, maxed ram, machine totally optimized for pro tools as per avid guidelines etc... should have been a no brainer.)
i waited until v10.3.4 thinking all the bugs would be worked out by now, which is totally not the case yet! (in avid's defence, i think some of the stability problems have to do with mountain lion... really not a big fan so far!)