What do you like best?
Really easy to set up and use, whether you're a site editor, front end dev or a hardcore PHP ninja. The interface is lovely too.
I'm front end, and don't have a lot of PHP knowledge, but I've built a number of sites using C5. The plugins are all approved by the guys who developed C5 so there's less risk of code clashes.
It's built on Bootstrap too, which is my favourite framework, so it's easy to convert a Bootstrap site to C5.
There's also a great community of really friendly, helpful developers who've helped me with any tricky PHP I've had to deal with.
What do you dislike?
I really wish more people had heard of it! I always end up doing a speech to clients and other devs because Wordpress is the one everyone knows about, but having used both, Concrete5 is by far the better option.
I wish there were more recent training videos. C5 has changed a bit over the years, and while there are plenty of videos for site editors, but I'd love to see some more front end focussed, or PHP newbie videos.
What problems are you solving with the product? What benefits have you realized?
Most recently I have created an online training site for a not-for-profit organisation. I've got jQuery multiple choice questions, drag and drop exercises and video/audio clips incorporated without too much trouble.
So many other features are baked into C5 and ready to deploy. A bit of CSS and you can really customise it to make your sites look great.
It's also open source and the plugins range from free to around £20 so I was able to keep costs down.