We had a tv show that aired a couple of weeks ago and it examined the effects of different underwear on mens testicle temperature throughout the day.
they got a dozen or so guys had them each wear a different type of underwear to wear each day and then stuck a sensor on their balls that took measurements every 8 minutes. they were able to compare the temperature differences between all the different types of underwear brief, boxer, jock, boxer brief etc and freeballing. it prooved that at the most the average temperature fluctuation overall was no more than half a degree celcius and not one of the guys (all had different jobs builders, office workers etc) during their daily routine got anywhere near the medicaly unacceptable temperature range. So I guess this prooved that either type are fine for your health but guys still have huge preferences for comfort.
Oh they also did the same test with a laptop on each of the guys laps and everyone of the guys temp went above the acceptable level within 45mins of having the laptop on their knee.
Since seeing that show I went and bought myself a lapdog fold up laptop desk thingy to keep my computer of my nuts.