I tried Al Rostimani in Dubai Mall recently and they pretty much screwed up everything they possibly could. They delayed the transfer (after claiming it hadn't been received), then transferred in USD instead of my home currency, at a much, MUCH worse rate than I had agreed with the manager himself (despite it being agreed in writing beforehand). After I complained they told me they would offer me a "staff rate" on the next exchange. That worked out to be almost exactly 3 dirhams different on a 20,000 dirhams transfer... so I cancelled the transaction and shredded my card.
Despite being a completely crappy bank staffed by morons Emirates NBD online transfers have actually worked quite well for me. As with all these transfers, larger amounts seem to get better rates and cost less on a percentage basis.
By all means give Al Rostimani a try, but I'd suggest a smallish transfer first just to test the waters.