Several people here are happy to use a teleconvertor with some of their longer lenses and most seem to stick with 1.4x rarther than the 2x as you only loose 1 stop as comparred to 2 stops with the 2x convertor. Basically if your lens is a F4 and you add in a 1.4x convertor your lens effectivly becomes a f5.6 and so would let in less light. With a f4 lens and a 2x convertor you would effectively be making your lens a f8 and unless you are using a pro body then you are likely to loose auto focus as well.
The same can be said for quality, decent 1.4x convertors can be bought that will have very little effect on quality were as it it widely regarded that a 2x convertor will degrade the picture quality more.
Makes and prices...I believe you use Nikon so obviously look at their own make but expect to pay big name prices or alternatively take a look at Kenko. If you buy the 300DG Pro version it is manufactured with top quality Hoya optics and the quality is fantastic. Just make sure you buy the Nikon version though as they also make the same convertor available with the Canon fit.
Hope that helps a little and if you have any more questions just fire away
