We've had lodgers for the last couple of years and it's worked really well for us.
We've sub-let two rooms to mature students, and I'd say this is a good thing to do - students are there for a reason, and they don't stay forever! This means if you end up with someone you don't get on with brilliantly (and you never know till you live together) it's not forever.
The most recent student we've had with us has been so lovely though, she's going to stay with us after she graduates and is coming with us to our new house!
My tip would be to be totally honest about how you live, and who you want to live with, when you're interviewing possible lodgers. So, for example, we're not the tidiest people, and we've learnt it pays to be really upfront about this. (Our current lodger is very laid back!).
Also stalk them thoroughly via google! (Particularly if they're coming from abroad and you won't get the chance to meet them). We've had a couple of lucky escapes this way - one woman who seemed lovely - but google showed us she was from a happy-clappy evangelical church, and has written on a forum that she was coming to the UK to study rehabilitation science because god had told her too
As committed atheists, I don't think we would have got on! The other was a single mum and her boy, they seemed lovely too. Until I found that she'd signed a petition online in support of a racist politician. Again, we simply wouldn't have got on!
That's another tip - you're not interviewing someone for a job, you don't need to be fair. You're checking them out to see if you want to live together. Do listen to any gut instincts or feelings that they're just not your kind of person. Be unreasonable!
Hope I haven't put you off! I really recommend it. We've made a few lovely friends this way.
And the money certainly helps!