BenchmarkForRails addresses a few blind spots in Rails' benchmarking:

• query construction costs
• before/after filters
• session management

See How Rails' Benchmarks Lie for more about the blind spots in Rails' benchmarks.

Other strengths:

• BenchmarkForRails.watch makes it very easy to benchmark new methods without implementing your own alias_method_chain
• automatically handles methods called multiple times (e.g. ActiveRecord::Base#find)

Installation:

Install the plugin from SVN (http://benchmarkforrails.googlecode.com/svn/trunk) however you normally would (piston, script/plugin install, etc.) and you're done. If you want to add benchmarks, look at the plugin's init.rb for easy examples.