I have to monitor our brand for my job, and one of the tools that I use are the twitter keword alert emails. I tried out a few a few weeks ago, but the only few that stuck out in the richness of features were Social Oomph and Tweet Alarm. After monitoring our brand for a while, I'm noticing that another facet to the "best twitter monitoring service" goes to the one who finds a given tweet out there in the twittersphere and lets me know about it fastest.
I'd have to say after having watching various instances of this, that the clear winner so far is SocialOomph. I receive email alerts from socialoomph a full 24 hours faster than TweetAlarm. Now that being said, I think Social Oomph could use a redesign, their website is hard to understand in some places, and the keyword feature is definitely hidden in their interface. TweetAlarm on the other hand has an extremely simple interface and I like the ability to filter out tweets from certain users (useful for brand monitoring). However, I have noticed that tweet alarm doesn't pick up some tweets on a particular keyword, and I'm not sure why that is as of yet. The only thing I can tell on some of the emails is that there is a comma after our brand name on some of the "missing" tweets, so that may be throwing off the keyword matching. So in essence wading through the complex interface of SocialOomph is definitely worth it,