1.) Difficult Deployment Equals Difficult Learning Curve: Add-on applications with long and complicated deployments will mostly result in a challenging learning curve for your users. Adding an application to Salesforce should not be as time consuming as deploying Salesforce itself. Look for applications which you can install yourself with minimal support from the vendor. This is usually a good sign of a well built application that is intuitive and easy to learn.
2.) Customer Reviews: Customer reviews offer some real value but I wouldn't place much weight on them. Sadly, some customer reviews are paid for, fictitious and misleading. One negative review for example can deter a prospect from evaluating a product that might be a solid fit for their organization. Don't be that person. Look past such reviews and evaluate the application and judge for yourself.
Have a great weekend!