Volumes 6 to 9 will give you many alternate versions of LotR, and if you're interested in the writing process yourself, then they are also interesting in that respect as they give a tremendous insight into the process of creating a book. Volumes 10 and 12 are favourites with me, as they include some of the stranger stuff, such as about Elvish reincarnation and marriage customs, and abandoned stories, which are fascinating to read.
If you have a look on Amazon, then you should be able to see a fairly good synopsis of what is in each book, but you can't beat browsing them in the bookshop, or even the library if you're lucky enough to have a branch which stocks them (like I did).
Did I enjoy them? Of course, as they have a load more Tolkien stuff in them!
But don't expect anything like the narrative found in LotR and The Hobbit. If you've read Unfinished Tales then you'll probably enjoy them, but if daunted I always say choose the ones which interest you most (at the risk of not understanding all the references straight away).