I like Chen. I think it take time to get a new character settled in and I think that she has generally progressed well. The only time I recall being really bothered was by one line of her's in "Greater Than The Sum". That line was, "Your Mamma was a fembot." I think she said it before she drop-kicked a borg on the E-E bridge.
As for characters that I would like see terminated:
Shar from DS9 relaunch...he is like Eeyore from Andoria.
K'chak'!'op from Titan...her character is meant to be matronizing...maybe for comic releif or something, but it is lost on me. She is a minor character, but a major nuisance in my opinion.
Fo'Hachensa from Titan - TV budgets limit how extreme aliens are depicted, literature should have similar restrictions on characters who talk like a Yoda knock-off. His lines always take me out of the story (I think recent books may have had him re-assigned off of Titan).
Dr. Ree from Titan has been declining for me overall in recent books. I think that may be because the whole Troi pregnancy thing dragged on so long. "Over a Torrent Sea" would probably be my favorite Titan book if not for the Dr. Ree/Troi sub-plot.