I've owned and used regularly a GNX3 for the better part of a year now, and I think it's terrific. Let's address your questions.
1. Well built - It's encased in a thick metal housing, kind of like a big BOSS pedal. Very sturdy (and very heavy!) All the knobs are small and well placed so you don't step on them, and the foot switches are very sturdy. No worries about the expression pedal, either. More metal, and it takes a licking.
2. Sound - Mine sounds excellent. It does everything I want it to. I have banks of sounds ranging from clean, pristine jazz, light blues, funk, to a very EVHesque lead tone, to crushing metal distortion (and LOTS of flavors of all of those.) The presets aren't too bad, but eh... I always program my own (The Eric Johnson and Santana presets are great, tho! Same for the Pat Metheney.) Lots of effects, all sound good, S/PDIF outs, 24-bit A/D-D/A converters, great stereo outputs, it sounds great.
3. Backing Up Presets - All presets can be backed up to your computer's hard drive via a MIDI cable (Make sure your sound card works with this, if you can. Most newer cards will be fine, or perhaps a USB to MIDI port.) On top of that, you can edit presets on your PC via the included GENEDIT software.
4. The 8-track is very good, esp. considering this is a pedal. It doesn't replace a newer dedicated digital multitrack, say, but it does its job pretty well. There are three recording quality modes... A. CD. Full 16-bit. B. LP. Slightly lower quality, more time allowed, and C. EP. Worst quality, longest time. For longer recordings, a SmartMedia card is a must, preferably 128MB. which is 24 track minutes at CD quality. I believe it comes with 16MB of on-board memory. The looper is terrific, too. All done on the fly with the footswitches. There is an optional Digitech footswitch you can get JUST for recording functions, too, so you can retain the use of the pedal's footswitch funtions for changing presets, etc.
5. I love mine, and wouldn't get rid of it, but there are a few other choices that work well. The BOSS GT-6 is one. Very comparable sounds, even more presets, similar layout, etc. Also very versatile, good digital quality. No looper or 8-track, however.
The Vetta Amp is similar in terms of function.... the new POD XT is basically a Vetta in a box, but definately more of a pure amp sim than say an effects processor. The BOSS and GNX have the advantage of being both amp simulators AND effects processors. Hope this helps. I'd highly recommend the GNX3 (unless your a snobby tube purist!)