Denis,
I haven't tested specifically with 3.x, but the 2.5 version likely works with 3.x already. I'd recommend trying it out on that version if you absolutely need it. I can't guarantee it will work and recommend installing on a test site first.. or testing a lot on your live site. There really aren't many changes to the underlying user system from 2.5 to 3.x though, so my guess is that you won't have issues.
If you do run into problems, let me know.. or if it works, that'd be great to. We do have plans to test and make any changes necessary for 3.x, but if we get feedback one way or the other, that will push us along quicker.
Thanks for the compliments, and glad you enjoyed the J1.5 version so much!
Alex