Is it ok to use Bittorrent on solid state drives (SSD)?

You don't download bittorrent data at SSD speeds (usually). Why not do bittorrent on the rotational media, and do low-latency / high-speed operations on the SSD? The only operation that would benefit from the SSD would be the reassembly at the end which, on the whole, is probably the smallest portion of the time required for the overall transfer.

That said, check out the lifetime of the SSD. It is probably in total number of writes per cell. As such, more writes means sooner death. If using it for bittorrent means you would use it more, yes, it will die sooner.

Note that this applies solely to receiving files via bittorrent. Sending them is another story entirely, and shouldn't affect the lifetime of the drive (since it involves only reads)

Hmm. Maybe.

Bitorrent has pretty heavy write requirements, and causes a lot of disk usage. On the other hand, modern SSDs do manage pretty impressive numbers of cycles before failure. I'd say it isn't going to kill it, but might reduce life somwhat

If you have one SSD and one spinning hard-drive, you can set (atleast in uTorrent) the client to keep uncomplete files while they are downloaded on the spinning one, and then move it to the SSD once it´s ready.... i don´t know if this will cause less wear on the SSD than writing it in chunks, does writing 1Mb 100 times use up 100 times more lifetime than writing 1 100Mb file? maybe something inbetween?

The SSD "wears" relatively quickly when writing small files. So 100 1MB files hurt the SSDs longevity. Writing one 100MB file on the other hand doesn't hurt it at as much. Just be careful with those rar or zip files where the movie split into several smaller files.

You generally don't want to keep movie files on an SSD though; only the OS, applications, and (possibly) games.