I was looking around and stumbled across this page, and read this:

Alright, the JJ ECC99 isn't NOS as it's still in production. Nevertheless, it's worth considering. Specs pretty similar to 5687 BUT said to be 3 times less noisy.

Which lead me here, and I eventually found the datasheet.

Any idea about how it would compare to a 5687 etc?

I haven't heard the tube yet, but various sources say it's similar to the 5687 but with lower distortion. However some have complained that it can be a bit lean and too solidstate-like. That's more or less the general consensus I get from diyaudio.com and audioasylum. I was going to grab some for my current amp project but ended up using 6BL7's instead since the local surplus store has a nice stash of them.

In theory it should be a nice tube, and at $12 a pop or so the last time I checked it's still pretty affordable.

Mikhail has built some adapters and tried the tube out. He said its a good tube but the 5687 is better. He was experimenting with a 12au7 gain tube with the ecc99 as the output tubes in a PPX3.