The answer depends on a number of things.
While it’s technically true to say that iPhone 3G owners aren’t left out of the iOS 4 fun, they kind of are. That’s because not all features of the OS work on the 3G, given its hardware differences from the iPhone 3GS and iPhone 4.
So, before you upgrade, you’ll want to get the lowdown on which iOS 4 features work on the iPhone 3G.
If that’s OK, it makes sense to learn from other users. Not all upgrades are smooth, and some iPhone 3G owners have had problems with the upgrade.
Ultimately the upgrade decision is yours – but maybe the bigger and better upgrade is a new iPhone?