It's slightly nicer than stock MySQL and I can more easily get Percona to help me if it goes bang (it has far more stats available than stock MySQL, for instance).

Some of my sites have sufficient write activity that using XtraDB (basically a slightly souped-up InnoDB) is a smarter option than MyISAM. For searching people can just use Google, it does a much better job than MySQL's inbuilt fulltext search.

I don't know if they've fixed it, but AFAIK tables with full text fields used to cause MyISAM joins to go to disk, even if the full text field isn't included in the query. As you can imagine, this sucks. Maybe that's been fixed.

We started using Percona's MySQL builds by default a couple of years ago. There are some nice performance and convenience features not included in the MySQL.com builds. (Un)fortunately we still have quite a few MySQL 4.1 instances happily running and are still using MyISAM across over 360 million tables. Most of that stuff is not running Percona.

Not sure in this environment, but I've been using Percona builds of MySQL for the last couple years and it's a great drop-in. Good optimizations for InnoDB Oracle-owned bits.