I've been using an ancient cellphone/external GPS combo running RaceChrono for the last 7 or 8 years now, and I'll speak up for that sort of cheapo setup if you're on a super-budget. It works pretty well - not as slick as the AIM stuff of course, but way better than you might expect for the cost outlay involved. We're talking under $100 bucks all-in here.
The program itself (unlike Harry's, apparently) doesn't seem buggy at all to me, and it's pretty easy to use. With my 5 Hz external GPS the lap times obtained are accurate to within a tenth (actually probably more than that, but my user-defined racechrono start/finish doesn't line up exactly with the real one). A more modern 10 Hz GPS would be even better. You can look at the data on your phone between runs, or export it to a PC later for more in-depth analysis. There are multiple options for doing that analysis, I used to use the PC version of RaceChrono, but Ken MacKay turned me on to using Racelogic's Circuit Tools 2 instead. Both are pretty slick, they can show you way more stuff than I ever bother to look at.
Your mileage may vary, of course.