Flew JU twice recently. CDG-BEG, and BEG-LHR a few days later, both in Biz. It's a lovely little airline.

Pros:
The food was great. On CDG-BEG I was served a Salmon dish that was cooked near perfectly. The BEG-LHR (breakfast) was some sort of meat pie, which was decent.
The flight attendants were very attentive and friendly.
They have a nice lounge at BEG. It's a smaller lounge (to be expected for an airline with just 21 planes), but has a decent spread (hot breakfast buffet when I was there), and attentive staff.

Cons:
Seats are now standard configuration for inter-European flights (all slimline seats, with blocked middle seats for biz).
Ground staff (other than the lounge staff) aren't quite as friendly.

I would definitely fly with them again.