D Gray-man is a very good manga. While the basic structure and character types don't break any moulds, Hoshino-sensei (who is a woman, by the way) obviously has a wonderful imagination. As such, the series is full of original concepts and ideas. Also, Allen is one of the most endearing shounen leads around. Throw in lots of bishies, great stylistic details and fantastic artwork and it's easy to see why the series has attracted numerous fans of both sexes.
The problem I have with it is that I feel D Gray-man falls short of its (very high) potential. The main reason for this is because the action scenes are so badly executed. Basically, you have no idea what's going on. Perspectives change with no warning, action from panel to panel is hard to follow, the drawings themselves are sometimes indecipherable. It just goes to show that artistic skill doesn't always a great mangaka make. In the world of shounen manga, these have got to be some of the poorest action scenes I've ever seen, not because they're badly drawn, but because the panels just don't flow, they don't make sense.
Admittedly, I haven't read far into the series so I can only hope that this aspect is improved on. Also, it's a bit of a shame that some people are put off by the slow start. It does get better.
At this point, I am buying/reading this manga because: I like Allen and am interested in seeing where his story goes; the side-characters are pretty interesting too (except Lenalee who is just a pain in the backside); and I can't get enough of the styling/artwork. However, there are some areas which need serious improvement. Hoshino-sensei really needs to work on her battle scenes, and there needs to be a stronger plot direction than what I have seen so far.
Overall, I would recommend this series to hardened shounen fans who don't mind swallowing some shounen stereotypes to get to a good story. The feel of D Gray-man is kinda like FMA so it's probably not fast-paced or simple enough for younger readers. If you're aren't really into shounen, then I can't see D Gray-man holding much interest (unless the lure of bishies and pairing potential proves too much for BL fans to bear).
EDIT: another thing that bothers me is Viz's treatment of the series. Maybe it's just early printings, but I found a few typos in some of the volumes. Also, the naming of characters is sometimes inconsistent, eg. Reever is called 'Riba' in one of the author's notes, and Lavi is called 'Rabi' in another author's note. ALSO, I don't get why Viz kept the Japanese-ified 'Arystar Krory'. Surely, given that in the manga the characters are supposed to being speaking English, this should be something like 'Alistair Crawley'? .... beats me