A big thumbs up!
I use the sidekick with my 200-400 VR and also often with my 70-200 VR too. It's great value for money when you consider the extra ease of mounting and handling larger lenses. I wouldn't hesitate in recommending it. I found that once I'd marked the balance points for my camera/lens * camera/extender/lens on the lens foot, I can set up the lens and balance it extremely quickly. Sometimes I do wish there was a slightly more secure clamp - removing a D2X/200-400VR can be a handful as the clamp is aligned vertically and you want to make absolutely sure that the plate is in the QR when you clamp it up or take it off. I'm not sure what would make for a better design than the screw clamp - maybe a VERY strong lever clamp perhaps?
If you are shooting with the 200-400 VR I'd recommend also getting the Wimberley replacement foot as well. I believe there's one for the 300 (it might even be the same). This is lower and brings the lens closer to the mount and is a really nice rigid assembly.
Also, there is a foot available for the sidekick to make it fit a bit better with Kirk/RRS heads etc. Personally I haven't found the base plate to be any problem with my Arca Swiss heads, although there's very little 'wiggle room' if you use an Arca QR clamp vs screw clamp.
I'll also recommend Wimberley as a company too - they have been really flawless in shipping out product to me as promised and the quality of the parts has been arguably as good or better than either Kirk or RRS plus very reasonably priced.
Is that gushing enough praise for them? (of course, your mileage may vary!)