As long as your on a continuing upgrade track, I'd say get it if you can afford it. I took a similar upgrade route as you. I started off with a Yamaha system much like your JBL's. It was a cheap way to get 5.1 sound right away.
I upgraded the center first to a Polk Audio
CSi30 , which really made me want to go out and buy the rest of the speakers that night. I think you will notice immediate improvements in dialogue clarity and presence. However, in my case it did sound a bit empty around the rest of the room comparatively. Especially when things in movies pan from Left to Center to Right or vice versa. However instances like that are nice in that they show you just how much better your system will sound when everything is upgraded.
But you gotta start out somewhere unless you're rich and can afford to buy everything at once. Not a bad speaker to start out on tho.
My Home Theatre
http://www.dvdfile.com/interactive/f...threadid=24525