Have used a few over the years - currently liking MailChimp for their strong API support (we integrate it into our signup process) and lack of forced branding in the outgoing emails. Many of the other services stick their logo at the bottom of your emails whether you want them to or not.
Depends on what you're looking to do, though, the different services all have strengths and weaknesses.
I use CampaignerPro (www.campaigner.com). Decided on them for cost vs. functionality. Pretty robust software with low cost for our volume (30K emails a month).
When looking for an email marketing solution you should make sure you determine your needs for:
- volume (how many emails per month will be typical?)
- reporting (how detailed do you want the reporting of the results to be?)
- customization (will you want a simple template you can edit yourself or multiple options for customization of the email, including dynamic content?)
- segmentation (will you usually email out to the same list or will you need to segment your list at every time?)
These are just some of the considerations for the email solution.