I would assume that they are only worth the amount of traffic (type in usually) that goes through them? Since it's hard to get redirects and parked domains in Google as it is, I would think it wouldn't do much unless you had thousands of them, or a very big, one word domain redirecting to a site. Does anyone have experience with a very good type in 301 redirect that helped them significantly in Google?
Apparently a 301 passes PR according to google - although the traffic is probably more important anyway.
A 301 isn't a back link, it is a traffic sign as such while PR really does not count for much when it comes to rankings.