My phone is an unlocked Samsung Galaxy S3 from Canada. Bought a Vodafone micro-SIM (20 EGP) from a shop in Cairo (Dokka) near my hotel. Signed up for a 2GB monthly prepaid data plan (90 EGP) and added another 50 EGP of talk time, which was optional. Caller ID, Voice Mall, and per-second billing was included. Total cost was 160 EGP or $25. A similar package would've cost me at least $80 in Canada! The Vodafone rep did the installation, testing, setting up the account etc all free of charge. Excellent customer service.
I was able to get mobile internet/data coverage in all the tourist areas of Cairo, Alexandria, Luxor, Aswan, and Abu Simbel and on the major roads in-between. Didn't work inside tombs but that is to be expected. My phone showed "4G" but I think the network is only 3G. Average data speeds, as measured by Speed Test.net app, were 2.1 Mbps download and 1.7 Mbps upload. This is much faster than the WiFi provided by some of the hotels that I've stayed in!
There's is no roaming for data within Egypt so this wasn't a concern. I was able to generate a hotspot for my wife's iPad and use the phone at the same time without any noticeable impact on browsing. After 3 weeks of browsing, Skyping, Google maps, FB, etc. I've used about 1 GB of data.
Having a local SIM with data gave me the freedom to be always connected without any concerns about roaming or overage charges. It became especially useful as some hotels in Egypt charge for WiFi. I found it to be very usefulR.
Hope this information is helpful.