We use New Relic but I've found it less useful than expected for performance tuning.
The UI can show you the slowest requests over the past hour or so, but those tend to be outliers involving some kind of cache miss. I want to be able to see typical requests.
If you drill down into one of those slow requests, it will show all the SQL statements executed and how long each took. We use ActiveRecord though, so going from an SQL statement to a line of source code is painful. I wish it showed the stack trace for each SQL statement.
Finally, it does not seem to have the kind of embeddable performance panel linked in the article. That would be super handy.
It actually appears that their tracking can be used free in development mode without their server; it could probably be adapted to be used on a live site without their service as well.
I've used NewRelic quite a bit in the past, but only in production. I never really though of using it in dev. It would be good if there was a browser plugin or something to give immediate feedback about the most request that loaded the current page, as shown in the article.
Just like the author of this blog post I was inspired by the MVC mini profiler and started a Ruby gem that has some very very basic functionality at the moment, you can find it here: http://github.com/bittersweet/stopwatch
It just shows the page load duration and amount of queries so far but I plan to have the same functionality as the original and GAE one :-)