* It's a physical space away from home. That helps create a mental separation which helps me focus.
* You're around people in the same grind as you. When I was working on a startup, I could share war stories with other founders.
* Leads - when I started consulting, I got my first couple leads from others in the co-working space.
* Signaling - if you're a business, it's important to signal that you run a professional establishment. Sometimes I'll book a meeting room with a new client at the space just to signal this.
Cons:
* They can be noisy, especially if you are around sales folks. I've only been to open seating spaces however so my view is limited.
* They can be costly. I lucked out here - I used to pay $400/month for a dedicated desk in NYC and I pay $150/month now for a floating desk. These prices are way below the norm for the area.
All in all, worth it IMO.