I want to second all recommendations of SwissMicros. I own DM15L (~$150), DM41X (~$220), and DM42 (~$250), and they are all absolutely excellent specimens. Not a single complaint. They are easily user serviceable, they have a very high build quality and feel like they will last decades (matching modelfkeyboards dot com build quality). The DM42 has a user-enabled N-stack so you can input algebraic expressions of any arbitrary length. The DM15L is an advanced calculator you (or at least I) can grok completely end-to-end, and packs it all into a compact form; I actually prefer to grab the DM15L before the DM42. The DM41X is also an excellent specimen. It includes the ability to {,un}load module files which are dumps of the old HP-41CX module cartridges. I love every single one of them, and cannot shill (for free) SwissMicros enough. You've beheld Swiss watches, now behold a Swiss calculator!