1) Is it doable? Yes. It's very easy.
2) Is it legal? If you haven't signed a contract stating you won't tether, then yes. It may be against your provider's policies, but again, unless you signed something stating you won't tether (and/or won't violate the carrier's rules), then it is legal.
3) Is it a good idea? That depends. First, see #2. I'm on a prepaid plan with O2 at the moment. Their terms and conditions prohibit tethering, but given that I at no time agreed to said terms and conditions (no, "implicit agreement" by simply using the service doesn't count), there are no legal issues for me. They might cut me off if they find out/care, but given that I don't use a lot of data, there's no problem. I've not found battery life to be an issue either, but I mainly only tether for short periods (such as needing to do some quick remote work in an airport, etc.)
Really, I've found that carriers don't care whether or not you tether, provided you're not using it as your primary connection. If you're tethering to play games, stream music, and download movies, then yes, they'll care. If, on the other hand, tethering is just a "make do" solution for when there's no WiFi, chances are they won't care, even if it violates their terms of service.