I'm running 10.04 alpha on 2 boxes, and have been happy with its
stability - I've never seen an alpha release with so few bugs, nor have
I seen an alpha release of any distro that had working nvidia drivers,
until now. There are a few minor nits, but I don't mind, as it's
definitely good enough for my daily use for email, web browsing and music.

As to your question, you do not have to reinstall. As long as you let it
do the updates, it will be automatically updated to the final release. I
started out with alpha1, and it's updated itself to alpha 2 and alpha 3
since then.