I just started up the retail version of the game and to my surprise my beta save was still available and it actually works. Should I just use this save or should I start a new game to avoid missing things or causing problems with the game?
I don't think the final version has been out long enough for people to find potential problems with old saves and adequately answer that question. I'd just export/import my character.
In the Warband 0.860 released thread Armagan noted you shouldn't delete your savegames,
since they would be compatible with the final release. So you should be able to play on.
If you're prepared to keep playing with your current character, keep playing. If it bugs out, you'll be right where you were before if you'd started a new game.
I just started up the retail version of the game and to my surprise my beta save was still available and it actually works. Should I just use this save or should I start a new game to avoid missing things or causing problems with the game?