DISCLOSURE: I work for Red Gate Software.

I've used all the major tools and most of them against a production environment. Each of them does different things for you in varying levels of detail and in different ways. I'll freely admit to a bias, but here are the three I'd evaluate:

Red Gate SQL Monitor (of course)

SQL Sentry

Confio Ignite

Our software is a good choice for general monitoring, up-time, guidance on performance problems, historical information tracking & custom metrics and alerts. SQL Sentry is good for highly detailed monitoring and nth degree drill-down, customizable alerting, and some fairly sophisticated tools for diagnosing performance issues. Ignite is excellent for identifying exactly which queries are causing you performance problems, some general monitoring, some alerting.

In addition to what these things do, you'll want to look at cost because for some of the higher-end products, you're paying higher end pricing.

If you do decide to evaluate SQL Monitor and you need any input, get in touch.