My wife, my son, and I all share the same wireless connection with cable internet access. I have two hardwired servers that run/support FAH, Web, and other activities. Even with downloading files, checking email, etc... everything runs smoothly. It is difficult to tell of any bandwidth loss at all. This is partially due to the frequency at which these things occur (not all at the same time, some of the time).
I do not think that it affects much if you use any open wireless. However, you are on their internal network. So don't abuse that aspect. (e.g. hack their computers, printers, etc)
Personally, I get a little bit irritated with people that have open wireless and default router passwords. I think that a person who owns a technology should know even a cursory amount of knowlege about it, if they are going to use it.
I played tag with somebody nearby me for two weeks by turning off his wireless in his router configs. I guess he could figure out how to turn it back on, but he never would change his default password. So I would turn it off again. Back and forth we went at this. FINALLY, he changed his default password. Yeay! I was kind of being a dick, but also... I just wanted the guy to lock his stuff down a bit for his own sake.
My advice... use it, but don't abuse it.
