I have used both. The L is the better of the two optically, but by only scant margins, as the non-L is deadly sharp as it is.
Is it worth all the extra money... hmm.
You get a hood with the L, and I think the hood is about a $40 item. You get a case with the L, again some cost with that.
The rest is the IS, and better build quality. The non-L is not built badly at all. The old non-USM one... er... not as nice, it grows with focus etc. But the USM non-L is a fine lens.
I would not justify the expense on the optical quality alone. If you can justify the IS... then by all means, its worth it.
The only thing similar I have is the 24-105 (similar focal length at 105), and find with some effort and deliberate concentration, I get sharp results fairly easily at 1/40-1/60 range of speeds. Keep in mind macro shots - very close - you're shooting darker due to magnification, and you also will show any motion blur more easily.
If somebody said - you gotta do the rings, but cant use a tripod, I'd use a monopod - which would be up to the task in almost all cases.