I'm a big fan of merino base layers which I use regularly in colder months and also when riding, normally under a Gore soft shell jacket or short sleeved jersey. I have tops from Icebreaker, Smartwool & Embers and love them all
I have noticed though at the pub stop after a ride they do hold onto sweat and take a while to dry out, it's never caused much discomfort though as the pub usually has the fire on to keep warm.
Just recently though I've tried a new "Core-Merino" base-layer from Embers which has a Merino inner and a Polyester outer, the difference in warmth (when stopped riding) and how quick it dries out compared to pure merino is fantastic.
If you're looking for a winter riding base-layer these are definitely worth a go. The fit is also slightly less baggy than merino due to a small amount of elastane.
My pure merino tops will now only be used for when it gets a bit warmer and other none riding activities.