Valuable features include creating and using defined templates with logos, and the ease of importing lists.
I would like to see reporting capabilities and graphing reports.
We have used this solution for over eight years.
There were no issues with stability.
We have a relatively set audience, which does not change.
I love their tech support. They have not been needed frequently, but when needed, they are quick and very knowledgeable.
We are currently planning to switch at the end of this year to a different ESP. One reason was that, with IBM's takeover of SilverPop, we did discover that customer care and our relationship/sales manager were no longer responsive or as caring as they had been in the past seven or eight years. The feeling is very different. We had never felt ignored because we are not super-users, but now, we do feel that way.
The initial setup was very complex for our company, as we only wanted to set up defined templates, which took some time, we have about 20 defined templates we created at the beginning (2008-2009) and don't need to use any others. So, it was complex at the beginning and very easy thereafter.
Because we have a defined (and relatively small) audience, our price has been very reasonable.
I do not know if other products were evaluated.
Take your time implementing, get plenty of training, and have clarity in your questions during training.