" when you realize how crappy an Atmel '51 is"

The parts are fine, it's the documentation (or lack thereof) and support (or lack thereof) that are dreadful.

Most chips, not only Atmel, has problems when they first come out. What makes Atmel "crappy" is the delay in issuing errata (as you say 'documentation'). You can live with many glitches in a chip if you know about them, however when you report a problem with version 'e' and the errata does not come out till version 's' (letters may be slightly inaccurate, long time ago) you lose all trust in a brand.

Erik