I'm around 11 stone and 5'8", and purchased a medium. Only worn it in the water once so far, so can only comment based on that. It's slightly hard to get into at first - a common thing I've found with Yulex wetsuits - but it fits perfectly once on and I'm sure it'll ease after a bunch of wears as did my Finisterre winter wetsuit.
It performs more like a 4:3 which is awesome; 2 hours in the July North Sea (13-14c) a couple of days ago with no flushing and didn't feel the cold once. Robust construction so should last, too. Aesthetically speaking, it's a lovely blue colour and the little details are a nice touch. Only criticism would be that I ended up with a naughty neck rash but that's my own fault for not wearing my BodyGlide or a rashy, and rushing out for a dawny. Would definitely recommend to anyone sitting on the fence wondering if it's worth it. As always, an excellent purchase from Finisterre.