I have a Mares 7 mil semi-dry (which I think has been discontinued). It has a drysuit-like zipper across the back of my shoulders and double seals on the arms and legs. I've been diving with it for about 3 years and have worn it in water temps ranging from the low 50's to low 70's. When it was newer, it was much more "semi-dry"--it would take about 30 minutes before I felt water run down my back. Now, not so much. I still like it because I think it is warmer than a standard 7 mil wetsuit (it has a built in core warmer), but when it dies, I'm not sure I will buy another one. I would say that if you don't want to invest in a dry suit, it is probably a good option, but how "dry" it is, will depend on how the seals hold up.
just my two cents.