I thought I would weigh in with my recent experience developing (not switching) a Prestashop site. Seem lightweight I know but I suggest trying it out, it's free.
I had looked into Prestashop not that long ago and they didn't appear "ready" but they have come a long way and I have to say I'm impressed with the ease of use, features and relative wealth of free features.
Now I haven't stressed tested this and my current development is for a handful of categories and products but I've read that it was tested with something like 100,000 products and passed. For a small or medium site it is pretty snappy speed wise.
It has semi-friendly (adds a category number to products/categories) URLS out of the box and I think a module to make it clean to help move your products/categories over (provided you have them already set for friendly URLs).
Lots of stuff included and many free modules but the money to be made is for some paid modules but I think many sites can do without the bulk of them. My current, albeit simple, developments with Prestashop have all been done with base package and a couple of free modules and edits of my own (none of which are very complicated or technical......'cause I is stupid

Now I haven't jumped off current Interspire and hoping for a little longevity but at this point I think Prestashop is where I would look to first for a package switch at least for my own personal needs. Still, not looking forward to it

Oh yeah, during these latest developments I actually looked at all of the mentioned packages and maybe it was just an initial impression but Prestashop stood out to me and especiallys for an open source package (sorry Zencart, loved you at one point).