Just be aware that at many (most?) hotels, the concierge has a network of vendors who provide him/her with kickbacks. That's not a problem if the vendors are good, but unfortunately they sometimes aren't.

In particular, we've had issues with restaurant recommendations. In a few cases it was obvious we'd been sent to a particular restaurant to facilitate a kickback, not because the restaurant was good. This was very common in Rome, for instance. We've had some horrible (and ridiculously expensive) dining experiences at concierge-recommended restaurants. We've actually gotten better dining advice from people on the street!

As a result, now we always research restaurants before we travel. Sometimes we'll ask the concierge to make a reservation at a particular restaurant of our choosing, but we don't seek recommendations.

We've had great experiences with concierges arranging things like limo service, though.

Tipping is totally discretionary and varies by region of the world. In Japan, a tip would be perceived as insulting. In the US, I think $10-$20 is a reasonable tip for something that doesn't require anything more than picking up the phone (i.e., making a reservation). If the concierge does something more complicated, i.e., wangles you tickets to a show that has been sold out for months, a bigger tip is in order. Tip at the time the service is requested.