I have a couple of Ultra-G's. Personally I think they sound pretty decent. Not as good as impulses, but to my ears, they hold up to many more expensive ones.
They sound different than most of the higher end Palmers and such, but at the end of the day, impulses sound better than all of them, so why drop the cash?
At $35, they have been immensely useful to me. I used them for silent recording or scratch tracks with my amps (still need a dummy load). They sound great with a bit of verb. Some guys prefer the clarity, especially blended with a mic'd track and phase compensated with PhaseBug or the LittleLabs IPB.
I even prefer it to a lot of "stock" speaker sims in modelers, especially a few years ago before many became impulse based. With things like the V-amps, I used the Ultra-G onboard the Pro Units all the time. Way better than the internal sims.
These days I use it for direct and zero latency monitoring of tube amps for tracking. I take DI track, a Raw amp Track, and the Ultra-G track. You can add impulses later or re-amp, but at least I get the feel of my amps. They were built to model the Jensen C12 cabs, which are a bit bright, so keep that in mind.
Hmm.. at oh 7 years I have had mine, and they have traveled a lot, thrown in gig bags, rolling around in cases of other DI Boxes and cables, they are beat up and still work. One of the B*hringer Jems...