Hi David,
I have a Vixen Polarie and am extremely satisfied with it. You can see a couple of results on my Member’s Page.
Clearly polar alignment is key. The polar sight has a field diameter of 8.9deg, I’ve attached a copy of a table from the manual showing the permitted exposure times assuming you can align on the pole to within 2deg.
If you intend to use longer focal lengths or much longer exposures I can recommend the optional polar alignment scope which is a very solid piece of kit.
Lastly, with longer focal lengths and exposure times you need a really solid tripod to mount it on. That much is obvious, less obvious to me was the need for a really tall one otherwise you end up kneeling on the cold and wet ground to align the device.
Hope this helps, if you want to ask anything else feel free to drop me an e-mail.
David