I have to take the counter prevailing view here - do it.
Effect on you: I did this years ago whilst doing many miles of bicycling and moderate weight training. Never felt a thing from it - certainly compared to donating whole blood which is quite depleting on your body. That you can do it 2x/wk week after week says something about how taxing it is. Also, if you weigh more than the minimum (I think I was almost 2x the minimum weight) the effect is commensuretly less as they don't take any more plasma from you if you weigh more.
Effect on Recipient: Have you seen where these collection centers are? College areas and skid-row. Either way your getting lots of alcohol and other drug metabolites in there. Personally, I'd rather the source of my plasma-based product be a bodybuilder with good nutrition and some extra Test floating around than from some ghetto crack whore (think hepatitus, birth control hormones, dirty crystal meth, and STDs), but it's not like you really have a choice when you in the ER or whatever.