@rjnagle: I started using PB just a little under a month ago. My trial would have ended in a few days. Because PB has met my needs and I've been actively using it, I went ahead and purchased a license for 5.0. The serial number is good for 5.0 and 5.5. In that sense, you don't really have to wait for 5.5 to come out to register. You can use the registration with either one.
I guess the only real concern is whether there would be a price increase or decrease when it does get released. Personally, I don't like playing those types of guessing games, hence, why I went ahead with the purchase.
Note: I have not used 5.0 at all during this period. Yes, 5.5 has had its fair share of bugs, some particularly annoying. All told, I found it quite usable for my full-time usage. And that's only gotten better. As Dryopithecus stated, the most recent release has been the most stable yet. YMMV.
If you are just starting out I would recommend using as much of the trial period, if not all of it, to get a good sense whether it will work out for you. Whether it takes you a few days, weeks, or a month to decide -- there's no real rush. When I was done with the trial period, I looked through the Core and Pro features, weighed their importance, and ultimately was able to make an informed decision on which version I wanted to go with.