Old modules are generally very playable (if you do the conversion). The higher the mod, the more work, though it also depends on the monsters involved (Dragons spring to mind).
I personally enjoy doing the conversions. It's fun (for me) to take 1e monsters and characters and see how best to reflect them in 3e terms. It's best IMO if you take key npcs and add prestige classes, feats, etc to make them appropriate.
One thing, if you are converting Basic or Expert modules, I would highly recommend subtituting monsters to give them more diversity and make encounters more appropriate. Remember that those modules were limited to the very small monster sections found in the Basic/Expert Rulebooks (Unlike AD&D which hada separate Monster Manual and several sequals). For example, you see Medusas being placed in several modules where there's no particular reason for them.