Depends what you expect from it-with these things price is a good indicator.
Might be OK with a tripod but I would avoid the x2 converter which will loose even more quality.
I noticed the filters are only for the small end that fits to the camera,so the front of the lens is exposed to the elements,therfore you may have to get a filter for that end 76mm I think it said.
A good filter of that size would be fairly expensive,so bear that in mind.
I have used a SIGMA 600mm Mirror lens before with 35mm film Nikon camera,and unless you have really good light,it`s of little use.
Still if you can get it cheap enough,it might be useful to you,but don`t expect too much.
You could always re-sell it if you don`t think it`s much good.