Calculators are terrible. I own an HP48 and a TI83 and despite their incredible power I find them both horrible to use. The simplest of operations can take many keystrokes and despite using both of them for years I never reached a stage when I felt I was an experienced user. I always spend my time hunting for the next key to press. They both have a plethora of operating modes and many functions assigned to every key. I was so annoyed by all this recently that I went out and bought myself a simple $12.99 Sharp calculator from Walgreens. While it's not a great calculator, if you want to do simple things it's much easier to use than the HP48 and TI83.
On the other hand, I miss RPN and 3 days ago I was reunited with my old HP32SII after 8 years. If you look on places like Ebay and Amazon you'll see that these machines are often sold used for over $100 despite being quite 'outdated' now. For most things this calculator is a joy to use. Even though I used it very little, and not having at all for the last 8 years, I was instantly able to dive into using much of the functionality very quickly. I still have a few complaints about it: with the 32SII HP had started playing with 'algebraic notation' side by side with RPN. This gives the calculator a bit of an inconsistent schizophrenic feel, especially when dealing with expressions ie. solving equations or computing integrals. It also has only a 4 level stack, a major shortcoming, especially when complex numbers take two stack entries each.
So I'm still waiting for the ultimate calculator to arrive. I've been tempted to build my own using a microcontroller, an FPGA and an LCD display - but I've been unable to find a cheap way to make my own keyboard.
Actually, what I'd really like is a pocket machine running Haskell...
UPDATE: I just got a HP42S for nothing. The size of a normal calculator, fairly uncluttered, yet with full support for graphics, complex matrices, equation solving and even sound.