Leaving the euro would be a good thing for Greece, who would then have control over their own currency so they can inflate away some of their currency problems. There is some danger in doing this though in ending up in a hyper inflation scenario which could take Greece decades to solve. No matter what Greece will still have to implement some austerity measures because it was the spending that got them into this mess and if they don't stop it or slow it down nothing will change.
The real problem if Greece leaves the euro is the damage it will cause to the rest of the EU still using the euro. Since it sets the precedent that any euro debt will not actually be backed by the EU and any individual country could drop out and repay their debt using a worthless currency, this will hurt the euro as a long term investment platform.
Greece leaving just he Euro would be more beneficial that leaving the entire EU, but that may not be up to them, the other EU countries may seek to remove them for the damage their actions will cause. Greece will likely see mass emigration as they deal with their debt, and remaining part of the EU will make it easier for Greeks to migrate further straining neighboring countries. No matter what Greece decides to do things are likely to get a lot worse before they get any better.