I just got my Alchemist head and matching 2x12 cab. It's a very good deal, imo. The cleans are pretty pristine, and with the flick of the deep switch and adjustment of the mids knob, you can get your desired Fender, Vox and Marshall cleans on the 40W channel. Headroom is pretty good on the 40W channel but I haven't cranked it past 1pm so I can't say for certain. The 20W breaks up sooner, of course, and the gain on this channel also has more compression going on. I like the gain channels on both 20W and 40W, tho the 40W does sound a little more open. I like the onboard delay and reverb efx - they really give that extra oomph in tonality that I'm looking for. My mum wasn't too happy when she saw the Alchemist rig in my room, but once I played it for her, she was convinced that it was a great buy. Hey, sometimes neutral observers can give you useful feedback too.
As mentioned by Bukka, versatility is a key differentiator for the Alchemist. The EQ knobs are very sensitive, and you can really use them to dial in a whole variety of tones, easily. There's a manual that gives you suggested knob settings for a whole bunch of popular tones, and I'd say they get you pretty close. Then you can adjust to taste. The boost channel gives a nice fat midrange boost, that pushes the amps overdrive into "searing lead" mode. V nice sustain with this amp. The amp also works well with both single coils n humbuckers, and retains the tonal qualities of individual guitars.
The build also seems very decent, despite all the MIC overhang. Doesn't feel particularly cheap, but of course I can see how it could be improved aesthetically. At least it doesn't say "Bogner by Line 6"
As a friend put it, "too bad for Line 6 that this amp doesn't say Line 6 on the front". Overall, the Alchemist is great value for money.
