depth of field plummets quickly. they can be helpfully but they can also be a pain to use. i've gotten many a out of focus picture from using them. but also some fairly intriguing ones
My experiences using them with 35mm SLRs hasn't impressed me. I think image sharpness suffers too much, although maybe I haven't had the opportunity to try out high quality brands of 35mm SLR close up filters.
When I got my Rolleiflex TLR it came with a Rollei close up lens/filter. I didn't expect much, but when I tried it out I was blown away. I've since bought several more, and I love them. So it may be that the higher quality brands are pretty decent.