I have had a BTX 85 for a few months and I really like it.
Some of my answers to these questions are not really relevant because this is the first scope I have ever owned, but I will answer them anyway.
Q. Does anyone uses the BTX as "main spotting"? How do you feel with it?
A. The BTX is my 'main' and only spotting scope. I feel great with it, it is a very capable optic. It reminds me very much of the Swarovski SLC 15x56 binoculars, but double the magnification. It is comfortable, it is packable, and it can really reach out there. I bought it (over a different scope) because of the binocular eyepiece and I am happy with my decision.
Q. Do you miss the zoom?
A. I can say that in general, I don't ever feel the need to zoom when I am using my BTX. Maybe this comes from my background as strictly a binocular user. Also, I do think there is value in the fixed magnification in that there is one less adjustment to make, which makes using the BTX pretty quick and easy.
Q. Do you use the swaro ME 1.7x? Is there a significant loss in sharpness? What about focusing with ME 1.7x?
A. I haven't tried the 1.7x booster yet. I may get around to it sometime in the future, but I am in no hurry.
Q. Does the size of the BTX module complicate the transportation of the spotting on the field?
A. I don't think the size complicates anything too much. I do carry a backpack that is mostly dedicated to carrying the scope and the tripod, but I wouldn't say it is complicated beyond what I imagine carrying a smaller scope would be like. Bigger scope = bigger backpack. Either way, its going on my back.
As I have said, I really like the BTX. It's a very cool optic that I hope to have for a couple of decades at least. The binocular eyepiece is similarly as comfortable as a pair of binoculars.