Fleece is of course polyester, and it works fine for many people. It can be made durable and wind resistant/waterproof, it's a nice option to have, light and generally available cheap. Problems: it's flammable and melts so be careful, it is hydrophobic so it can get clammy next to skin, odors are not pleasant as stated, and it doesn't insulate when wet very well. I like fleece ok, but I'd say I've transitioned about 90% to wool. Smartwool or woolpower next to skin, and go from there. I know its durable, I stay dry, and warm, and when you compare them in the woods, wool absorbs light and is much more muted than fleece. I use down if I need more as a mid layer. IMO the more experience I've had with wool, the less I wear fleece.