Is it worth going to preview event for accepted students?
If by "admitted (accepted)" you really mean "those who accepted their offer," it's just a nice, welcoming gesture. During course work, you'll work so closely with and next to your future fellow students that you needn't worry about falling behind socially if you don't attend.

If people have been admitted but still weigh offers from different schools, it's a marketing event in which faculty and current students will try to present their program - and the campus it's on, and the city and state it's in - in their best light. While it would certainly be nice to get to know other prospects, it is probably more helpful to use such events to get a feel for how you might get along with professors and such, and if the city is too busy - or not busy enough - for your taste. 

If you're not rich with extra time to spare, it's probably only necessary to fly in from Europe if it's case 2, and you have genuine questions or concerns. These can be nice events, but I'd think twice before flying for 6-10 hours each way on your own money - as long as you can't combine it with a longer vacation or so.