Hi Rider -

I have the Manfrotto 488RC2 and it works great. It has a full range of motion and is pretty sturdy. I've had it in all sorts of positions to frame a shot and the biggest issue I have is keeping the center of the camera field aligned with the scope axis so that I know what it is I'm pointing at. Takes a couple of minutes to dial it in sometimes.

No issues with ball head creep - the knobs tighten down very snugly and I've never had an issue with the position slipping. I use an Canon40D with the biggest lens so far being a 100mm f2.8. I just ordered a 200mm f2.8 so looking forwards to using it to capture the birds and stars, I'm sure I wont see any issues with the ball head in this configuration.

I do sometimes have problems with the 3/8 to 1/4-20 adapter ring at the bottom side - the head is tapped for 3/8-24 so it can be used with a tripod and there is an adapter ring that you can use for 1/4-20 bolting. In my case I have it bolted onto the top of one the rings of my guide scope, which has some counter bores for 1/4-20 bolts. I use the ball head for day time photography a lot so I'm taking it on and off constantly. If I forget to really torque the adapter down it sometimes slips. Good thing I always remember to set the strap of my camera so that if something slips it wont fall off the scope. I'm thinking of getting a second head just to avoid this, but I'm also thinking of using the counterweight shaft of my mount to hold a camera.

I've got a couple of ADM accessory plates and they are very well machined and finished. I have the VDUP-11 as the main plate under my Pentax scope and its nicely made, and I think the plate I have under my guide scope is the Vixen plate you mentioned. I'd recommend any of the ADM plates and adapters highly, its a nice surprise when you receive something that's well made and is at fair price - I do a lot of work with local machine shops and I couldn't get any of the ADM parts made with the same quality and cost as ADM offers.

Cheers

Mark