Always DIY. I paid painters once, and they did a terrible job.
Quality paint will save you time and money. I buy either Behr Marquee from Home Depot, or Benjamin Moore. They're more expensive per can, but you'll need fewer coats to get good coverage, so I find that the price balances out.
Like Starbuck, I don't tape any more, as I find that I get better lines by just edging with a high-quality brush. If there are two of you and one is better than the other at edging, then the other can work a roller while the other edges - my neighbour and I did her whole house this way, and it was a snap!
Keep a damp cloth handy, so that you can wipe off any spots or drips before they dry.