As far as telephoto lens go- yes spend the cash- those 40 dollar ones in stores are hit and miss- one I have works fine with one cam but with the other I get halos and vhs like quality when zoomed in rendering it pointless. The Sony High Grade 2x lens that retails for 100 bucks is great.
I've bought two non-OEM telephoto lenses - one for my Panasonic and one for my Sony cams (it fits both of them). I don't see any difference with either when I use them - even in full zoom. But I've seen video where the edges of the picture got very blurry when the filmer was using a zoom lens and he zoomed in all the way.
If either of the lenses I purchased did this, I simply would have sent them back. Fortunately, they work great.
One area in which I will not use non-OEM parts is with long life batteries. There are a lot of shit batteries made in China that don't come even close to living up to their stated battery life. A lot of them are advertised as true Sony, Panasonic, Toshiba, etc. batteries on eBay - but they are cheap rip offs.
Note: I heard the new Sony HDV cams won't let you use anything but a true-blue Sony battery.