What's the quality of sony brand name filters and lenses? Can I assume mid-of-the-line, higher then "digital optics" (the ebay brand), but lower then Crystal Optics? (I am refering to regular sony lenses and filters, not the "profesional" ones)

Don't know if you can generalize. They make some cheap adaptors for their 37mm camcorders that sell for ~$30 at Best Buy. Those are just cheap junk. But I got their "high grade 2x telephoto" with a 37mm mount which was expensive but produces fantastic results on my PDX-10.

On my VX-2000 I have their .7x wide adaptor (also not cheap) and it's really nice. The only real drawback is that there are no filter threads.

What I'm wondering about it something like this 58mm filter kit:

http://www.bhphotovideo.com/bnh/controller/home?O=productlist&A=details&Q=&sku=200957&is=REG

Could I expect it to be of better quality then the no-name generic brands?

Those are reasonable filters. I think you can judge somewhat by price on filters. Reputation is a better guide if you have some history in photography or videography.

I use B&W whom I think are one of the best if not the best filter supplier. But I buy sony filters for my travel camera. It's a matter of application and expected life.