It's not a gain monster, but it can get heavy without a boost. I have one, and I used it in the industrial/thrash/death band I was in. It behaves much like a Marshall, in that you really need to turn up to get the most out of it; it won't sound great at low volumes like a more "modern" high gainer. There's easily as much gain as a channel-switching JCM 800 (2205 or 2210, which have much more gain on tap than the single channels).
They won't do 5150/Recto style high gain. The amp's not voiced for it, and if you want to get heavy with it, you pretty much need the gain at 3:00 or higher on the red channel. There's enough there if you don't need super-saturated gain. The low end can get kinda flubby on Fluid Drive, the highest gain setting, but a boost will tighten it right up. I use an SD-1. As far as heavy tones go, it's probably more suited to Alice In Chains/Tool-type tones than what most of you guys are going for. I'm betting it would get great Opeth tones, though, as their sound isn't dependant on extreme gain.
It's pretty agressively voiced, especially in Bold mode. Tone-wise, reminds me of the bastard child of a JCM 800/JMP Marshall and a Bogner Shiva. Tite Gain and Fluid Drive modes have just enough gain on tap for metal once you turn up.
The Series II Stilettos have been extensively re-worked and are much higher gain. The Series Is are not a bad amp by any means. They're well built, have cool features- selectable power/rectifiers for each channel, an effects loop that doesn't blow like the Rectos, etc. but they don't have near the gain most metal players want. I like them, but I wouldn't recommend them unless you can live without the usual modern metal gain/voicing.