I lived in Allen about 2 years ago before I got my house a little ways down the the highway in Murphy.
Allen was okay but I really didn't like it that much but that had more to do with the fact that I missed living in Irving, which was a lot more D/FW-centric.
Allen is a really nice place to live. It has a pretty strong school system, football is king and there are tons of places to shop. Also, it is just north of Plano, so you have tons of parks and trails (Plano is known for its trail system) to go to as well.
The only real downside is that traffic is a beast. You have to pick your poison with either Highway 121 or Highway 75. Also, they don't have public transportation, so if you want to take the train into Dallas, you will have to do it in Plano or another city. So if you have to commute to Dallas (or Gawd forbid Fort Worth, like I am currently doing), then be prepared for mornings and evenings in traffic.
When I was looking for a house a few years ago, it came down to a house in Allen and the house I eventually got in Murphy.
Also, they have really built up the area off of 75 and Stacy road. They have had the Allen Premium Outlet out there for years but now they have all kinds of stores and restaurants on the east side of Stacy and 75.
Also, you might hit up City Data's forums for more information about Allen.
http://www.city-data.com/city/Allen-Texas.html
Hope this helps.