"Sequencers" is a big topic. There are many types of sequencers and very different uses and strong opinions (Metropolis and Tirana II are pretty much at opposite ends of the spectrum). This has been debated extensively at Muffwiggler, and I can recommend reading through the threads over there.
A quick search gave the following places to start (but there are many others):
http://www.muffwiggler.com/forum/viewtopic.php?t=216839
http://www.muffwiggler.com/forum/viewtopic.php?t=161153
http://www.muffwiggler.com/forum/viewtopic.php?t=180779
There is also an extensive comparison of sequencers here:
http://doudoroff.com/sequencers/