I stopped here today on my way to Berlin and visited both the Mikwe (there is an aboveground viewing window) and the old synagogue, with a small museum inside. Contrary to the other reviews here, I quite appreciated both the good fortune that led to the main building surviving, as well as the dedication that went into rescuing it, while still allowing all its uses over the centuries to be visible in the stone and wood. 

Was it "big" and "impressive" like others? No, sadly not, and I do think the SchUM sites offer a more comprehensive look at German-Jewish history. However, I am glad it's preserved and honored, even if it's a smaller site than others.  

That said, I would still quip that the entrance price (8 Euros) is a bit much for the size. If you want to avoid that you can see the main back stone facade from the side street, so it's easy to visit and check off for free if you wish.  

Erfurt's old town is quite lovely and is worth visiting anyway, and this is not out of the way to visit at all.