I had the Bionic and I loved it -- well, I loved all 4 versions of it Verizon sent me, as the damn thing kept breaking (and/or it wouldn't hold a signal, and rather than admit the problem was on their side, they kept sending me replacement Bionics until I gave up and demanded something else). They replaced it with a RAZR Maxx (to which I never had any sort of emotional attachment), and I just switched to the GS4 on Sunday.
Conclusion: This is not a huge step. It isn't like when I got my first smartphone (OG Droid) from a stupidphone. Nor is it like when I got the Bionic from the OG Droid. (Am I missing one in between? If I am, that felt like a huge step too.)
This is a smaller step up. I'm starting to think that, now, I won't be getting new phones every 18 months/2 years, because the huge improvements in the phones aren't happening so fast and furious. This phone is more refined, and does a few more things than the Bionic -- although many of them (like all that wave-your-hand or move-your-eyes business) either don't work all that terrifically or are more of a toy than a tool. Fun to play with, but not changing-your-world stuff.
I like it and I'm glad I did it, but I don't think it's as necessary.
One thing which may make it weigh in your favor, as it did for me: VZW doesn't really like the Bionic, and have been crazy slow in pushing updates to it. They're probably going to stop that altogether, if they haven't yet already. The GS4, being a flagship phone, is something VZW is going to keep current for the next couple of years.