Jacen was always my favourite EU character. I was just at the right chronological age to read the YJK series and I associated with his character a lot, enough to form somewhat of a lit-crush on him.
Flash forward to last year when I started picking up the NJO books and I decide to do a quick wookie-search... imagine my surprise when the Jacen Solo page redirected to Darth Caedus! I was heartbroken.
From a literature perspective, I was somewhat disappointed with how his character arc was handled. The jump from YJK-Jacen to early-NJO Jacen was disjointing, though I brushed it off; there were bound to be characterization and perspective shifts between the two series, from the different authors, to the vastly different storyline and form of storytelling, and also simply because it was a jump from a teenager character in a YA novel to a teenage character in an adult novel.
Within NJO, I think Jacen was handled very well. His early indecision may not have made for the most exciting of reading material, but it made Traitor all the more potent and memorable, and helped to built up to an amazing culmination of the series. The fangirl inside me couldn't help but cheer him on the whole way.
Then.... DN came. Another sudden, awkward characterization shift, and this one much less forgivable than the beginning of NJO. Looking back, I can see it all as setup for LOTF, but at the time, my brain was just stumbling trying to pick up the pieces. I couldn't even lie to myself and tell myself it would all get better, since I knew Darth Caedus was looming in the future, even if I didn't know the where, why, and when.
I actually found myself cheering Jacen on in LOTF. By the time he started down the path, I knew there was no going back so I Just jumped right in with in. He was shaping up to be a good villain, one I hoped would be a major driving force during this next Star Wars generation. Instead, his fall and death were way too short.
I guess this is the point where I admit I pretty much cried all the way through Invincible. The jokes at the beginning of each chapter were a poignant reminder of the old Jacen, the one I read and loved as a young teenage girl, such a far cry from the dark, brooding Sith. At this point, I didn't even want Caedus as an epic villain; I just wanted my Jacen back. Of course, neither of those things were to be. Jacen may as well have died back at the end of NJO.
Looking back at more of an emotional distance, I still think Jacen could have been an amazing long term villain. I like the longer story arcs that Del Ray has been bringing to the EU, but I think there's a lack of overall cohesion that Caedus could have filled as an antagonist. Ideally, if I were in charge of the Galaxy, I would have spread his fall over a much larger time frame, filled in the gaps with smaller books and series here and there, and saved the longer series for a grand stand-off of some sort. This also would have had the benefit of spreading things out so it didn't feel quite so rushed.
Anyway, this post already kind of ended up longer than I meant it to.
tl;dr: Jacen was a good character, but DN/LOTF rushed through him and spat him out like bad wine. I'll miss him a lot.