I liked Concrete5 when I was playing around with it. I was very excited by its potential; especially looking at projects where maybe WP was overkill.

Where the bottom fell out was in two areas:

1. Paid plugins

I cannot rationally tell my clients 'this software might have a few cases where it is easier than WP to use and manage; but you'll be spending several hundred dollars replicating free functionality available on WP'.

Its a non-starter.

2. The software still looks kind of 'hokey' compared to WordPress

WordPress has lots of issues and there are a ton of other great alternatives. But, its like Facebook vs. all other social networks -- you go where the people are because it makes everything better.