Hi Jim,
Well, maybe I can shed a little light. While I'd not strictly from an airline background per se, I did fly scheduled for a few years and have experience of both sides a little.
1. What are the main differences in lifestyle that one can expect? My personal circumstances in the job right now... I hardly ever get up before 10am, when I do, it's the occasional early morning and that's about it. I rarely get home later than 10pm, when I do it's the occasional night flight. I have around 20 days a month at home and when I work I tend to do a flight and then settle down for a sauna followed by a nice meal in a 4* hotel. I get paid reasonably well, regularly go to around 30 different destinations, get to see a few odd ones in my travels and I thoroughly enjoy the job. On the other hand I have thousands of less hours than my equivalent airline friends, but have the joys of Russia, the Middle East, Africa and the America's to play with. Having said that I had cultivated (cultivate) contacts to get me into the good jobs, which is very very hard to do. Downsides, my telephone is never off. No such thing as saying NO. They call, you work, simples.
2. Is this an advisable move? (I have not heard of many people in my airline making this switch!) - It's advisable if you want to be a biz jet pilot. But remember this, you WILL be doing LOTS of Jeppessen updates. You will be cleaning the interior, making coffee, lifting bags, having to be extremely flexible, sitting on the plane for 5 hours delaying a flight plan waiting for a punter who's perpetually '20 minutes away'. You'll also be doing your own drop line loadsheets, your own paperwork and possibly your own slot coordination and even potentially filing flight plans.
3. As an FO, what are the general chances of being upgraded? (perhaps lack of proper career progression structure compared to an airline?) - Upgrade tends to be face-fits coupled with ability (ability is not just techs and no-techs, it's front of house customer service). Passing a command assessment is irrelevant. Standards are all. Do you keep high personal presentation standards, a clean cockpit and a spotless cabin?
Hope that helps shed some light on your questions.
Regards
PW