Really good sounds, useful presets, LOUD!
VOLUME: First things first: this dude is LOUD!!! I guess I wasn't really thinking straight when I went for the 75-watt version simply because it wasn't a big price leap from the smaller one. It didn't really occur to me that this beast is 3/4 of a 100-watt face melter. This is not a giant problem and it's certainly not a "fault" with the amp, I just want potential buyers to be aware of the serious output on this amp. Unlike a tube amp, where you can really damage your tone if you don't push the tubes, and therefore you're better off not getting more wattage than you need, this all-digital amp will sound just as good low as cranked. But the tricky thing is keeping it under control as you change from sound to sound. I keep this on "2" pretty much all the time when playing through it at home. 2.1 hurts. No, I mean HURTS. If I have it dialed up just a tad on a clean sound and switch to a hi-gain, it can sometimes knock me backwards. So, this is a gigging amp, in my opinion, not a practice amp, simply because of the sheer output. Enough about that.
SOUNDS: You've probably watched the video. You may have played through other LINE 6 amp sims before, like a Pod. This is very much the same, in my opinion. It's not really better or worse than anything else LINE 6 puts out. Which is certainly very good. Until buying this to use for some live performances, I have been using VST plugins to do all my amp work. I've got a giant collection of sounds in Amplitube 3 and Guitar Rig 5, and I used to have the Behringer V-Amp 3. The Spider sounds better than Amplitube or Guitar Rig, hands-down. It has a lot less self-noise, and it saturates the tone in a "real world" way (I can't REALLY call it "analog" I guess) that is just more satisfying than Guitar Rig (which in itself is better than Amplitube, IMO). It's not particularly better than the V-AMP 3 for DI recording to my ears, but this is a combo amp, not just an amp sim sans speakers. If Behringer still made the V-AMPIRE, which I used to own, I probably would have just bought another one of those.
UI: The user interface is pretty easy to wrap one's mind around, though it's not 100% straightforward because the FX knobs are three FX on a single knob and there are also some "hidden" parameters accessible through other means. I don't fully understand it yet, to be honest, because I haven't cared to mess with it much. Each amp type also has two variants, distinguished by a red or green LED on the amp selector. Like me, you can choose not to bother much with any of that if you just like the factory presets, which I do.
PRESETS: Sort of a combo of the "sounds" and "UI" aspects, the presets are easy to access via the triumvirate of D-pad, knob and up/down switch on the right of the amp. There are some unsorted ones, some sorted by song emulation (which are cataloged by decade), and artist presets -- some of which I found a bit surprising simply because the artists are not that commercially known. Meshuggah having a preset was cool enough; to find that Minus the Bear has presets in this amp was downright mind-blowing. And among the 40 zillion presets on this, there are quite a few in different styles that sound really, really good to me. I'm sure that most people will have a similar experience. All these amp sim devices, whether hardware or software, always offer me way too many options, many of which either sound so similar I can barely distinguish or which are so esoteric as to not be useful. But as long as I can find 10 or so "go-to" sounds that I can use all the time, I feel like I got my money's worth. And the Spider IV has at least that many that I really like.
Ease of use and functionality would both be expanded even more with one of the optional foot controllers, and if this band that talked me into joining will ever actually play a freaking gig, I might invest in one of those as well.