Engel... Engel... Engel... I like it; it has a great sound and presence to it. I think that it would be the best compliment to use it as a nickname for longer names beginning with "Engel-", though. Vocalized or written, Engel strikes interestingly and attractively. It sounds very sturdy, but also has an elegance to it. In my opinion, Engel better suits males, though I would not be opposed to seeing it in feminine usage.
I quite like the name and its spelling. It's apparently quite unique in Germany, and it's and people looking at it will likely almost immediately associate it as being masculine. A very nice alternative to its more feminine-seeming counterpart, Angel.