Let's see... I had an E 35mm F2.5 lens once. It was a black one designed for EM and looked pretty ugly on my silver-chrome FM. The pics with it were fine, just slightly softer in contrast compared to the ones took with Nikkor lenses of the same length. One thing I didn't like was the built quality of the lens. I could easily knock it and that would be the end.
I also had an E 50mm F1.8 pancake lens which I got off Yahoo Japan auction. This came with the smell of the seller literally... It just smelled like someone's unwashed clothes or living room carpet. Anyway it was okay to use, but the helicoid part was already a bit loose, and I was just not happy with it, so I got rid of it. The pics took with it were fine in B&W, but in color, they were not as good.
So, I replaced them with the AiS version of the (Nikkor) lenses, 35mm F2.8 and 50mm F1.8. The AiS 35mm F2.8 was really good and much better than the E Series one, but it was not that small or light, so I didn't get to use it as much and got rid of it. I wish it was also a pancake size or somewhat close to it, but obviously it wasn't. Meanwhile, for something slightly bigger, there are good F2 versions (non Ai, Ai, AiS, etc) of the Nikkor lenses out there and they are just far more practical.
The AiS 50mm F1.8 is really good, too. This one is a pancake lens. I have the Japan-only model which focuses down to 45cm instead of 60cm and is built slightly better than the "export" version. And it's multi coated. I mostly shoot B&W with it, but it's so good and sharp. It tends to flare a bit in some situations, but it's the design nature (big front element) of it that causes it.
I like this compact size and its built quality altogether to use it on my FM. And this is the one I will definitely keep for a long time.