The Sidekick will "do",... but for the 600mm?,..
...a lens that large is what the full Wimberly is really made for.
I have a Sidekick and the el~ cheapo Mnafrotto Gimble,. ($165.00 with QR release and plate) and the Manfrotto is definately the better of the two.
The issue is bottom mount Vs. Side mount,. the heavier the lens,. the more important bottom mount is for balance and secure fitting.
Currently there are a host of Side mount options,. the Side kick,. Kirk Kobra, Mongoose,. and some others..
There are only two bottom mount options,. the Wimberly and the manfrotto.
For a 600mm I'd go straight for the full monty