I am of the (debated) opinion that the Merlin and the 5d are not friends
The combination does not have enough 'rotational inertia' to make a nice flying experience
You can do little walkies and runs that blow away hand held
if you try more complex moves, involving acceleration, stopping , lock offs or panning you are in for a real challenge
(note the Merlin promo vid does not show a lot of footage with those moves even on larger cameras)
The CMR blackbird is a product similar to the merlin that is a little bigger with the weights more spread out that appears to perform far better - remember that the rotational intertia of a mass is quadrupelled as you double its distance from the gimbal
Even the BB is a challenge to use in comparison to a heavier larger rig such as the Steadicam Pilot which the 5d is too light for unless it is in STEEL rails/mattbox/mic configutation - the Pilot needs some extra mass to make the arm work properly
Of course the Pilot is more costly and much more annoying to put on !
In terms of lenses I think 20-28mm range are optimal
None of these solutions offer the ability to focus which when matched with the 5d DOF creates problems
I like the Merlin, have balanced many different lenses on it (I have listed 9 or 10 Merlin Recipes over at Cinema5D.com), including creating a way to turn the camera on it's side and shoot vertically! However, I do find that it frequently loses it's settings and needs to be rebalanced. I suspect that this is because it is very slightly slipping on it's mount. I have done all kinds of things to shore it up including various gaffers taping tricks on the underside of the camera, but to no avail. So that mounting stand goes everywhere with me!