If you read HN then you understand the benefit of reading users comments after you read an article. Foursquare lets you read users comments while you are visiting a location.
However I am not sure if/how this translates to profitability so I am not speaking to that end.
And, of course, I still like earning new badges.
edit for additional note:
If you have friends that use it heavily while on tours it is fun to track their travels. Follow an entire band or tv crew on tour and get anecdotes from different personalities live updated as they travel - it's like a real reality show in small doses.
I've noticed I get lots of little freebies when I check in via foursquare. Ie., $5-$10 off a meal if i use my amex, a free dessert etc. Lots of local businesses seem to have these hidden deals but they don't publicise them.
"Only" 20 million users? "Only"? The HN commentary is an interesting place these days.
When I looked at some business reviews on Foursquare (while traveling— was in Washington, DC area) it was disconcerting how a large majority of reviews seemed to be from 2009-10.
Any HNers with an optimistic view, and/or use Foursquare and think it's awesome?