I don't know RPN and don't feel like learning, so the HP calculators weren't my cup of tea.
I originally bought one of the TI calcs that is allowed, since I adore my TI-89 and have always used TI calculators, but it just didn't work out well. Specifically, it was the TI-30XS Multiview and it SUCKS. It doesn't do much and doesn't have an intuitive button layout. I use it for keeping a running total of my purchases while grocery shopping, and that's about all it's good for (IMO).
I picked up the Casio fx-115 ES and really like it. Not only was it cheaper than the TI that I bought, but it does a heck of a lot more. I will buy another as my backup for the test.