I think they're great if you're using them for specific purposes. I find working at home almost too easy and it takes a long time for me to really get into "work mode" vs having a space somewhere I start switching gears on the way there and then feel more productive right from jump.'
Better work/life balance.
Lots of the small stuff is handled for you.
A real address for meetings.
Learn about other things going on in your sector.
Can't really tailor the details to your specific needs (volume, potted plants, artwork, etc)
You incur a substantive cost on the books of your company.
Potentially distracting if you're direct sharing space.
Rarely do they deliver quality on any extra-stated promises (Money connections, quality events, etc) so you can really only bank on having a desk.
I find the best alternative is to try and rent some space from a larger company that is growing/scaling so you get to learn a bunch, meet people who are focused, and usually you sort of join the family and get the benefits of free lunches and happy hours.