there is no catch. their business model is not on repo count, but collaborators count. in other words, solo devs and very small teams can freeride, if you grow bigger you start paying.

also, it's backed up by Atlassian.

I wouldn't say Atlassian is an asset. Jira is horrible.

For me Jira is great, apart from the fact that our corporate install is quite slow sometimes it's the best bug tracking software I've used.

I take great issue with that statement ;)

You can.

BitBucket is run by Atlassian, their goal is to later upsell you to their line of products, so they loss lead with free unlimited private repos in the hope that if you need a project management/bugtracker system you might choose JIRA.

It's not as popular and the site isn't as responsive (UI wise) as Github. But other than that it's pretty awesome. It supports Git for some time too.

No catch, I've been using it for years, and always wondered why people preferred github.