Well known for their stripy armed base layers, Helly are now pushing their new mid-layers featuring Lifa stay-dry technology.
The top is designed to be worn over a base layer, sweat is taken through the fabric to the outside - this fabric doesn't absorb moisture, it just passes it through, which feels odd when your back is wet on the outside, but dry on the inside. For stop-start riding it's not so effective because the moisture will sit on the outside, potentially making you cold, but if you are constantly moving it works brilliantly.
I've been living in this with a short sleeved Helly Dry layer underneath and the combination is great. For winter riding it's a great combo with a shell or waterproof over the top As Helly say, the new breed of mid layer really does put the fleece out of business...