Postgres has bitmap index support baked in as well and it works really well. I'd be awfully interested in knowing where MySQL beats it by 50x at anything.
Some MySQL vs PG tests done by Domas (of Facebook & Wikipedia fame) show some interesting differences between MySQL and PG for an in-memory read only test. I don't think there are any 50x differences though.
The tough thing about comparing these two is that it is hard to find a benchmark (I don't know of one) that works similarly with them both. Sysbench is somewhat of a standard for testing MySQL configs but Postgres does not handle it well.
Though I tend to prefer Postgres don't get me wrong - MySQL has some advantages.
For raw pkey lookups, especially range selects against pkeys with lower amounts of concurrency InnoDB's speed is unmatched. It should be given that it is completely laid out on disk specifically for that to the detriment of other features.
MySQL's replication is extremely sturdy. On paper the older method (statement-based) sounds incredibly fragile but in practice it works extremely well and has been proven on countless projects. mmm makes it almost too easy to setup replication and failover.
I feel that MySQL stalled out for several years in the 5.0-5.1 period where poorly engineered features were bolted on to create a product with too many pathological cases that destroyed performance to keep track of. All these features were available but experience taught you to avoid most joins, avoid most subselects, avoid most usages of views, etc.
That said, v5.5 has a lot of nice improvements and can actually scale up to more than 8 cores (I've gotten linear improvement up to about 32 cores and that is what Oracle puts in their white papers as well). Percona and Facebook are releasing nice patches and branches, and forks like Drizzle are reaching GA and doing good things as well. So I think it is headed in the right direction again.
Compared to MyISAM? Sure for certain aggregate functions, but it's too easy to point out reasons for avoiding MyISAM: fragmentation, no transactions, table locks on writes, complete table rewrites to change columns or indexes, only scales up to about 8 cores.
Compared to InnoDB? You are incorrect, Postgres is faster, even for a raw count(*), even when comparing against the InnoDB plugin and not the ancient InnoDB builtin.
I have access to tuned TB+ DBs of both types and am happy to disprove any specific examples you can provide.
http://dom.as/2010/11/08/random-poking/