The Seaskin 150 undergarment is an excellent value for the money. Especially being made to measure.
It feels "puffy" when you gear up, before you get in the water. But, once you get in and the water squeezes the air out of the suit, the 150 does not take any extra amount of lead compared to other equally warm undergarments.
I have the Seask 150 and 250. I wore the 250 when diving in Lake Erie, at 43 - 44F temps. It was so warm that the next day I changed to the 150 and was fine. I haven't worn the 250 since.
More recently, I have switched to Waterproof MeshTec 3D shirt and leggings undergarments. I think they are not QUITE as warm as the Seaskin 150, but still very warm and maybe more comfortable to deal with when gearing up, waiting to get in, and during surface intervals. But, the Waterproof undies are quite a bit more expensive than the Seaskin stuff.
In all cases, I wear the Seaskin Tech Base layer underneath and it is excellent. A bit warm on its own, and excellent at wicking moisture away from my skin.