Except if you want to do a nice pagination interface over a set of data. select count() in postgresql takes forever on large tables. select count() over myisam is speedy. select count(*) over innodb is speedy but an estimate (I'd still prefer an fast estimate over exceedingly slow accuracy for many scenarios, such as paginated web interfaces).
That's because of PostgreSQL's MVCC (Multi-Version Concurrency Control) architecture. And, yes, while some aggregate queries can tend to suck on very large tables (for which there are a good half-dozen workarounds; this is the FAQ-est of FAQs), the upside of MVCC is that read queries can never block write queries, and write queries can never block read queries.
The benefit that has in terms of increased concurrency is worth far, far more than having to implement a workaround for quick-and-dirty row counts for simple things like pagination, IMO.
(Aside: InnoDB is also MVCC-based, which is why its COUNT(*) is an estimate. The MySQL folks apparently decided that it was better to provide an estimate than an exact count, while the PostgreSQL folks decided the other way. There's a part of me that wants to call that symbolic of the way the two projects operate on a much broader level...)