Thanks for your input
Actually, reliability is about as important as looks (or rather, performance).
The thing is, if rendering is too slow, it will completely miss the point, and I'm afraid sw rendering only will be too slow since the applet has to be like 800x600 with potentially quite complex scenes. So, software rendering is a nice-to-have, but I'm afraid it will be not be enough by itself.
A pop-up saying "This applet doesn't run on Opera" or something would be better in this case than having a bad user experience. OTOH a browser crash for 20% of the users for example is big no-no. Webstart is also not an option, it really has to be an embedded applet.
So I think it has to be primarily a hardware accellerated applet, the most reliable option, which is not too big to download.
I know I'm kind of vague here, but I'm not at liberty to talk too much about it yet
I'm currently leaning towards jPCT somewhat, but I haven't ruled out the other options just yet (maybe even a non-java solution).