Camera+ actually shoots better pictures than the phone app--IF you know how to use it, and if you understand proper exposure techniques. It frees you from the forced default exposure of what point you focused on, and allows you to use another exposure point within the frame. This means that if you know what you're doing, you can adjust your exposures by the equivalent of a few f-stops--and that's a damn big difference to anyone who actually knows how to use a camera. It allows you to go from mediocre snapshots to possible works of art (or utter crap, when you screw up...LOL).
Of course, if all you ever do is point and shoot and let the camera do all the work, the built in app is fine.
The filters in Camera+ are fine, at least you can adjust them a bit, which helps. But it's the ability to have that second point for exposure that has made it my default app since the iPhone 4 first came out. (and it's what I miss when I use Hipstamatic for playing around with)