A couple years ago my co-worker and I each built the $22 IKEA standing desk from iamnotaprogrammer.com[1]. We also each got a nice standing mat and stool (about $100 each), and I got a monitor mount (about $50).
It's been excellent. I'd say I spend about half my time standing and half sitting on the stool. For me, rather than losing focus when I'm standing, sometimes I'll just pace around my office a bit, which I've found helps me think a lot, and obviously helps keep the blood flowing and the muscles loose.
[1] http://iamnotaprogrammer.com/Ikea-Standing-desk-for-22-dolla...