Wulff is right, neoprenes are very hot in the summer. However, you don't really have to wear waders in the summer if you can take slightly cold water temps.
I have neoprene waders, and come April I was starting to get uncomfortably hot when fishing. I'm really big into hiking/camping, so I decided in lieu of wearing waders, to wear my water sandles (keen newports), hiking gaitors and quick drying shorts. This provides you no protection from the water, but gives you sturdy footware with the sandles, protection from tall grass/thickets with the gaitors, and quick drying clothing.
It may not be ideal for everyone, but if you have the gear and aren't a wuss about cold water, it beats buying another pair of waders.