I think the question is: What do want to accomplish?
For a subset of features e.g. an activity stream for your WordPress Network it works fine in my experience.
If you want a full-blown social network, with messaging, events, groups, blogs, forums all working neatly together better invest a good amount of time.
Some caveats I’ve encountered:
- Lack of good developer documentation.. but it is getting better and there is support.
- Some if not most external Plugins are full of subtle bugs and of dubious quality. So be careful and check functionality before relying on external plugins. But this is also true for WordPress
- Updates (either Buddypress or WordPress) will break things in subtle and strange ways.
Some tips:
- Build the system in baby-steps, always check what impacts a change could make and test if everything is still working.
- Use memcache object-cache for speed and also apc! (but beware of object-caching bugs, take a look at the trac) once set up, buddypress is reasonable fast, without these rather not so.
- xdebug (tracing, profiling) & a php-ide are your friends for debugging
- have fun 🙂
I think it is great as an social addon for a wordpress network install, for a complete social network with lots of functionality i’d look somehwere else.