I've been to a restaurant five times this month.
I've been at a restaurant five times this month.
I've been in a restaurant five times this month.
I guess any of these is acceptable, but you're getting into the nuances of the language here. If I had to choose one, I'd go with "I've been to..."
You don't say "I've been in Rome five times", you say "I've been to Rome five times", right?
Even if at/in is incorrect (but I'm not sure), it's not a grave error (unless it's on a test on prepositions :)