How well does that work with mysql 5.5 in 2012?

Exactly.

(It's not actually simpler when query execution over 100s of millions of rows is a perf bottleneck, and each tenant has several billion rows in the main tables. Then you're grateful you can schlep them around, and keep small tenants fast, etc. Even now, Postgres would still be a dubious choice due to the unpredictability of its query planner, though I use it for all my hobby projects.)