The one thing I always liked about Opera as a developer is that they used IE as their spec. That is, if a site rendered correctly in IE, chances are it would do the same in Opera. The upside is that you'd only ever need cross-check for Opera compatibility a couple times a month (versus several times per day for IE/FF/Chrome), and it would generally still be working.
The downside of course, since they have such a small installed base, is that if ever your thing stopped working on Opera, you didn't have much incentive to spend the time and effort do do anything about it.