TBH as it's a free bit of kit, best bet is to kick the tyres on your server
The community is very active, way more active than anything ISC ever offered, with lots, and lots of mods out there, and a decent amount of guys who know their stuff.
Lots of mods are free, others paid - paid ones by and large are a tenner or so. The coding model is different to ISC, so takes a bit of getting used to if you're wanting to add in things that you've already modded. The 'vqmod' system is absolute genius IMO - it's an addon itself, but free, and allows you to make addon code that sits seperately to core files (XML file with all the changes as find/replace/add instructions - see here
http://code.google.com/p/vqmod/), so changes persist during upgrades, without any compare diff ball scratching. Majority of changes I've done to our sites use vqmods.
The mod store is very good - none of the forum bollocks of ISC, but an actual shop (the creator, Daniel, takes a cut of every sale) - so any mods you do, you can add there and get paid for, or give away.
I've not got any real specific answers for the Qs you've posted, though variations is better by a long way, I've a mod ($10??) that takes care of multiple SKUs for variations (also does a lot more than that).
Gripes that I have, that are probably easily solvable with free, or paid mods (I've just not bothered looking, lol!) as follows:
- Product filtering in backend is a bit pants - no easy way to search
In fact, no easy way to search orders or customers either
No built in import/export (this is a free easy fix, but daft omission)
Erm, nothing else leaping out actually?
Pro points
- Nice start layout
Good built in banner system
Easy to set up new layouts
Built in rewards & affiliate systems
Good checkout process
Good image handling (mostly, finding image in media manager can be faffy, again, fixable, just never bothered)
vqmod system is insanely good
I guess one 'drawback' is that I'm very used to using Drupal installs at the minute, and it doesn't have the flexibility of a full on CMS, but you can tag the 2 together (no official modules, booo), or there are some surprisingly powerful CMS mods available - just added a news one to one of our sites, butchered it into a recipe section, works a treat.