After 2-3 days of golf a week in record hot summer heat last year, I noticed several of my light-colored shirts provided very minimal sun protection - I got tan lines of the sports bras I wore underneath.

Thought I'd give this shirt in white a try since it says it's UPF 30.  Wore it the other day in 93-degree weather, and I think it's the perfect shirt:
  - kept me cool:  breathes very well and I think covering my arms kept me cooler than wearing a sleeveless shirt
  - blocked the rays:  I didn't feel like my arms were crisping up in the sun, and I didn't notice any tan lines
  - very flattering:  was concerned that the white may be too tight a fit (and very see-through), but the small was perfect - the right balance of loose yet fitted.  I did wear a white sports bra that had a few grey stripes, and the grey stripes did show through a bit, but not that bad

** 5/15/2013 update - in a hurry to dry the shirt after hand washing, I put the shirt (slightly damp)in the dryer for about 5 mins, and it shrank a bit. :(. Don't make the same mistake...
** 7/12/2013 update - still love this shirt (very polished and flattering), but w/90+ degree hot, humid days, this knit shirt gets hot.  (Or i feel hot because it is hot.). There are other shirts that do a better job at cooling.  And by mid June, noticed that my arms have gotten very tan (only 30 UPF).