It's impossible to toss all OVAs under one umbrella, as the usage for the term of OVA is pretty broad. Sometimes OVA means just extra scenarios that couldn't make it into the anime, like the Angel Beats OVAs. Other times a series may get totally remade in a non-standard fashion and gains the title of an OVA series (such as what happened with Hellsing). Sometimes OVAs will simply be used as a continuation if an actual new season wasn't approved, like what is still happening with the ever slow releases of Kenichi: The Mightiest Disciple, or what happened with Tsusbasa Reservoir Chronicles (both examples where OVAs do the series justice).
Are there good OVAs? Tons. The biggest problem with the most standard way you see OVAs is that anime sites and the like won't place an OVA chronologically in the anime timeline. Many OVA episodes, as I mentioned, are scenarios that don't make it into the anime. It seems a lot of the time, that doesn't just mean content that happened after an adaptation ended prematurely. That may just be a scenario that happened say, between episodes 4 and 5. It loses a lot of impact if we finish a series though and then see these OVAs that don't have nearly the same impact as the ending of the show.
Basically, when you know a series has OVA episodes, verify where they fall in the timeline first. For most anime, somebody, somewhere, will have compiled a list of what episodes fall where. They should be more enjoyable if you watch them when the events are at their peak relevance.
