Hi everyone,
What a timely topic! I've have just been through the exact same process. Here's what I found.
Bottom line is that I went with the DataSprings "Opt in" module and so far am happy with it.
In general, here are some of my thoughts and purchasing guidelines that I work with, not just for DNN modules, but for software in general.
1. I don't buy any software from a single developer company. Although it pains me to betray my heritage of also once being a single man software company, I need to have to reassurance of a company being more than just one person, for the reasons of tech support, product development and sales service. With this in mind, I steer clear of companies that are either named after a single person, or have a low "bus factor" vibe.
2. As a developer, I tend to gravitate to solutions that are resemble the way I might I might write the software. After reviewing all of the modules listed in this post so far, I started to be troubled over the fact that none of the solutions provided a list of "opt-in" lists that users could subscribe or unsubscribe to. Each seemed to inherit the native DNN group types, of which "Subscriber" seems the most relevant. (see the issues with this in the first post of this thread by Rodney) Perhaps the other modules I evaluated did do this, but if they did, they did so in a way that I missed it. The Data Springs "Opt In" module made it clear from the beginning and it seems that the rest of the newsletter module is built around this basic, fundamental concept. This design concept resonated with me. The ability to subscribe and unsubscribe to these lists made a big difference to my evaluation of the product.
3. I always read the product documentation, as well as the on-line support forums/knowledge base (if any). After reviewing all of the documentation of each product, I can say that they ranged from almost non-existent, to terrible, to excellent. The Data Springs module has good documentation, and a good on-line support channel.
4. Module maturity. Version 1 software is rarely any good. Version 1.3 isn't much better. I look for software that has been around for awhile and has a good user base. A good user base shows that the software has sold well, and I can rely on the community base to keep the developers honest and working hard.
5. Price. You get what you pay for. I believe that a lot of DNN modules are undpriced (I'm not alone on this one) Doesn't it stand to reason that if a software company is able to employ sales, support and developers, then they need to charge a fair price for their wares that reflect the design, development and the huge effort that goes into producing commercial software. Paying a decent price for a decent product show a company maturity.
If anyone would like detailed feedback on any of the newsletter modules mentioned in this thread so far, please let me know and I'll be glad to elaborate on my findings.
Hope this helps someone.
Best regards,
Rod