I don't mind. Flattered I guess, but it varies depending whether or not I'm interested.
If it's repeated flirting from the same person it's a bit different, for example I'm pretty sure one of my friends has been coming on to me, if I can determine whether I'm correct in that belief I may try to make something of it, since I'm into him myself.
On the other hand, one of my managers at work has been flirting pretty overtly for a while, and while it was flattering at first it's getting a bit much, bordering on sexual harrassment, so I may ask her to cut it out. Politely.
If it's repeated flirting from the same person it's a bit different, for example I'm pretty sure one of my friends has been coming on to me, if I can determine whether I'm correct in that belief I may try to make something of it, since I'm into him myself.
On the other hand, one of my managers at work has been flirting pretty overtly for a while, and while it was flattering at first it's getting a bit much, bordering on sexual harrassment, so I may ask her to cut it out. Politely.