I've owned Surge for quite a few years.
I bought Surge as a replacement for Z3ta+, which I found I was missing after switching to OS X. In particular, I missed the ability to modulate the wave-shape the way Zeta used to. Surge is not as flexible as Zeta in this regard, but it's nice to have some of those features back. I also liked the wavetable capabilities.
That said, I found that I really loved the straight saw sounds out of Surge: they are "crispy"... and while they have a slightly digital feel to them, I really like the tone. Dial up the "Happy" patch for an example. (And though Surge offers three variants of tone toward the top of the interface, I never truly found this tweak to make a big enough difference to be worth using.)
Surge is also adept at handling a myriad of other "digital" sounds... particularly FM. Though this won't replace FM8, it does have some great capabilities in this department, making it my second-favorite FM synth.
The presets are good... though they are not going to make you jump out of your seat, they cover a lot of territory, including basic sounds and really far-out stuff. Claes did a good job, here.
The UI is reasonable. Strange colors, a blocky, windows-like feel... but puts most of what you need in a very small space. I don't especially enjoy editing the FX, however (and by and large, I don't like the FX compared to other synths). Modulation routing is brilliant... one of my favorite synths for this kind of editing. Modulation in general is Surge's trump card: great possibilities with strong LFOs, good envelopes, a decent step-sequencer, and other tricks up its sleeve. The two synths that I find Surge most comfortably compares itself to (at least that I own) are Massive and Largo. ...another two wavetable synths. I own both, and I find myself reaching for them more often than Surge largely because of their UIs. Largo also sounds better, in general (better filers, particularly with drive), and is more flexible. Massive has a larger bass presence, but sounds much more digital than Surge. Massive's modulation routing is also one step up from Surge's, since you can see destinations immediately rather than having to click on each source. ...That said, Surge is capable of modifying the same destination with more modulators, if you need that sort of thing.
The sound of Surge is, overall, good. Not stellar, and it has a hard time competing with more modern synths. As a result, I've really stopped using Surge much. I still have a couple of weird sounds waiting to throw into a track here and there, and I certainly haven't uninstalled it, but I generally don't find Surge capable of keeping up with its competitors. I think it sounds about as good as Massive (though not for bass, where obviously Massive wins). Largo sounds better (again: it's the filters), but when you put Surge next to the most modern synths (Diva, ElectraX, Dune, Zebra, etc)... it's just not as "sweet".
That said, there are a few "weird" lead sounds that I really love in Surge and would be hard to reproduce in other synths ("Back to Tyrel", for example), and will probably make me keep it installed moving forward. ...I also just find Surge "fun". :) It's a synth with an attitude of its own, and I respect that.
Lastly, I'll say that of all the synths I own (about 20), Surge is in my top three for the number of patches I've written for it. It's a synth that invites editing.
I'm glad I own it. ...But I'm afraid I would steer people away from it, unless you're already well-stocked with the better modern synths. If you're looking for something quirky to add to an already-robust collection, then I heartily recommend it: Surge has a character and "spunk" that I'm sure you would enjoy.
FWIW, you can hear Surge presets being used in some of Infected Mushroom's more modern stuff, to give you an idea of how professional it can sound. ...if you like that kind of thing.
I should also mention that I've never had Surge freeze or crash on me, even though I've certainly put it through its paces.