I recently returned from Dahab and did my diving through Desert Divers (DD).
This was my 3rd visit to Dahab, and my 3rd time diving with DD.
As EgyptDiver said, there are 67 dive centres and so you should choose carefully.
One of the problems I see is that the staff at a lot of Dive centres are not long termers. This is true of DD as well.
Only on one of my dive trips with DD was an Oxygen cylinder taken.
This trip, I noticed that my depth and air gauges were inaccurate. This could've been dangerous as I'd be thinking I was shallower than I actually was. The unit was telling me i'm at 5 metres when we're at 10 metres. So if I had been diving at 20, I'd actually have been at 25-30. I told the staff and I was promptly issued a new unit. Guess what, same problem.
Now, I don't think this is a specific problem to DD, but probably to a lot of rented gear anywhere in the world. Just so you are aware. I was lucky to be diving with buddies who had proper dive computers of their own.
One thing that I wasn't too pleased about was that they forgot to pick me up from Sharm airport. I think that is a pretty aweful thing to do to a customer. I was given a free dive at the end of my package, so I'm not bitter about it.
This could perhaps be a warning to you, if you are expecting things to work like clockwork. In Dahab, they almost NEVER do. Be prepared for delays, confusions, et.c. Usually this is not a problem, but if it affects the safety of your diving then it would be a bad thing.
I would say they are the cheapest of the lot, but you get what you pay for!
Negotiate a taxi from Sharm airport instead of arranging a pickup as they will charge you more for pickup than a taxi outside the airport will. Look to pay around LE 150-200 to Dahab.
Stay at Alaska camp, basic, clean and cheap! 5 Euros for a room with a fan. more if you want a/c and ensuite. It's very central but very quiet somehow! I stayed at Crazy Camel to start with, but it's at the back of the two most popular night spots, Tota and Rush, so you hear a thumping bass late into the night. Otherwise nice camp.
Your best option, if you can arrange it, is to hire a dive guide independently. HE can then arrange your diving, tanks, taxis, et.c for all dives. Otherwise you are at the mercy of the dive shop, who will often wait till they have a few numbers going before pulling their finger out. I met a good Egyptian Instructor, so next time I go, I will arrange all dives through him, rather than any shop. This also meant that at some dive sites, where you'd usually expects hoards of divers, because of his expert timing, we were the ONLY people diving and ended up having an exclusive private dive! Highly recommended.