Aren't there other ways to do database replication?
Yes, there are a lot of third-party replication solutions for Postgres (http://wiki.postgresql.org/wiki/Replication,_Clustering,_and...), which offer various features (some of which offer even more complex and redundent replication options than the built-in replication). However, since they are all third-party solutions, there are such varying levels of complexity and support I think you would prefer to use the built-in versions of replication.
And as discussed in the post, I believe that streaming replication is a very nice replication solution, and probably good enough for most applications.
I'm sure there are places online that do a much better job comparing them, but from my experiences with Postgres I would have no reason not to use it for any relational database needs.
Depends on what you need replication to do for you. If you just want a full replica of your db, the built-in stuff is getting really good with 9.x (as far as I've heard; I have no direct experience with it). That said, the PostgreSQL hackers are specifically making most-case tools, and for most people, they're great.
If you have more complicated needs, though, you'll need to look elsewhere. The built-in tools are all-or-nothing, and single-master. I've worked in multiple environments where we had needs that went beyond what they'd have offered, had they existed at the time; multi-master, replicating a subset of your database(s), and transforming the data in-flight during replication are real needs that organizations find themselves facing, and based on what the core committee has said about their plans, those aren't going to be addressed with the built-in tools any time soon, if ever.