Some would argue that Glee ruins the originals and their favourites have been destroyed, but I would argue that it introduces many new fans to the original artist or song because they heard it on the show and went looking for it. In that respect, Glee has been a great promotional tool for many singers whose songs have been shelved long ago. In my opinion, if you like the original song, however you feel about Glee's rendition, you should be pleased that their cover has given it some much needed attention again.
For this album in particular I think it was a great start to the show. It was missing a few great songs, but overall I love most of the tracks on it. Standouts for me would be Don't Stop Believin', Taking Chances, No Air, Keep Holding On and Defying Gravity. Sometimes you can't improve on the original, but you can appreciate other versions.