I personally wouldn't want to take a girl to McDonald's as a first date. I usually have enough money to cover a meal from a restaurant that's at least a bit classier than McDonald's (and if I didn't, I'm sure my parents would help me out if I really needed it). I just feel like a first date is an important event, and it should be special, but that's just my personal preference.
However, a McDonald's date could be just fine. After all it's more about who you're with and how you're interacting with them rather than where you are. I might prefer something a bit more "fancy", but that doesn't mean it HAS to be. I'd much rather hang out with someone special at McDonald's than not hang out with them at all because I couldn't/didn't want to take them somewhere else.
Basically, it just depends on the person. If she's cool with it, and wants to spend time with you, that's great! Go for it :D