It’s partly offtopic, but partly it’s about captchas. Because you are talking about different ways to fight with spam. Akismet is one of real antispam methods.

As I wrote in article Fight with comment spam, there are different ways to block spam in comments. Some of them effective, some not - and from different methods I can mark out the most reliable: authentication services, CAPTCHA (secure ones) and Akismet. These ones have their peculiarities, but they are more reliable then others methods.

About Akismet. It has some issues:

1. Requirement of WordPress.com API key. It’s better to make people’s life more easy without spending time on different keys.

2. It’s not bullet-proof - Akismet can sometimes put legitimate comment to spam and it can let pass some spam (I saw such cases many times at my friend’s blog).

3. It’s third-party service, so you depend on them (on stability and quality of service).