I purchased the Kokatat Tropos Whirlpool bibs this winter and love them.
Kokatat Watersports Wear - TROPOS Whirlpool Bib
They mate really well with at dry top. I took a swiftwater rescue course two weekends ago. We were standing in the water for a long time, my legs never got wet.
I won't say they are as dry as a drysuit, but I think for the money they are as close as you can get. I wear a NRS Mystery shirt under my drytop and it gets wet (due to the age of my drytop), but my legs stay dry.
The only thing I don't like about the bibs is there is no relief zipper. You basically have to take off your vest and drytop to drop the bibs down.
I've been in contact with Kokatat, they can add a relief zipper for $100 and I'd be without them for 2-3 weeks. I'm going to wait until the winter to do it.
The Kokatat Gore-Tex bibs come with a relief zipper (and booties) already installed, but they are $500, so you'd almost be better buying the NRS Drysuit for that.