I'm using
the Onyx 1640 with Cubase SX3. They work fine together. I haven't had any problems and I've been using them for about 10 months.
The pre's on that thing are superb for the price. There is no other way to get 16 pre's of that quality for that price. That's why I chose it. I don't know how they did it because everything else on the board seem high quality too.
The only awkward thing about using the Onyx board for an interface is you can't use it to send channels to outboard fx gear and back DURING mixdown because, while there's 18 channels of input, there's only two channels sent back out of the mixer (which, of course you would use for monitoring). And you can't use the nice EQ or inserts on that board during recording because the signal is tapped for firewire before the it reaches the eq's and inserts. (Not that I would use its EQ if I could anyway, as I have better EQ that I use.) The DA / AD converters are nice too so you don't lose any noticable quality when you are using the 2 outs for sending something and then returning it. You know for recording outboard FX, parallel drum compression, etc to a track(s), re-amping and all that good stuff.
So to answer your question... yeah, the onyx board works fine with cubase. I just thought I'd let you know about the no-outboard-fx-during-mixdown issue, because some people get that board not knowing that and end up dissapointed. If you're going to use plug-ins during mixdown, you'll be fine. It works great for me.
Sorry for the extra ramblings I just thought I'd let you know about that other stuff in case you didn't know already.