In reply to Up High:
The meco wicks better than pure merino, dries quicker but still retains the anti-odour properties of wool (although not quite as smell proof). It's good for higher intensity activity but doesn't really offer any noticeable extra warmth over synthetic imo. Personally I don't like it but it's well reviewed, the only benefit to me over synthetic is the anti-odour. If the fit/colours were better I might wear it more in the summer but it doesn't get worn often.
One of my colleagues finds it scratchy and mine was bobbled and lost it's label after light use (not that I'm particularly bothered but all my other base layers have been hammered and still look newish). It's quite a loose fit for RAB as well, I'm not sure if their stuff is getting more towards high street sizing or I'm just wasting away but I used to a find a medium a good fit from them and the small in their base layers is pretty blousey on me.
I wear a thick merino top (icebreaker, £40 for the 250 crew) for work as I'm standing around in the cold all day and I really notice the difference, warm and comfy (and easier on the washing, 2 weeks easy but will still be fresh after a month and beyond if you're so inclined). For any outdoors activity in the winter I use helly hansens merino tops, durable, non smelly, the right level of warmth for me and a close fit. If you're on a budget then they're often on ebay from people who've bought one online only to find it's too small.