Just for another data point. I've stuck with 8.04 due mostly to laziness and the fact that it's working fine. I use the stock rt kernel. The only real tinkering I've had to do is build jack-rack from source since the stock ubuntu 8.04 version won't read or save racks. I get down to around 11ms latency in jack and I'm happy enough with that - I never notice any delay even when I'm playing guitar live. I know there are some cool new features that I'm missing and especially I know that I'm using a version of jackd that Paul has said is awful, but it works for me and as someone else mentioned, at some point you've got to be making music.

Regards,
Bill