I would encourage you to ask the vendor for a current client and then apply for a job through the ATS so you can see what the candidate experience is like. So many ATS systems are insulting to clients (upload your resume, now paste all that data into our forms, etc.). I believe a poor ATS results in some good candidates refusing to jump through the application hoops. Just my 2-cents...hope this is helpful.
My company has been with iCIMS for the past three years. They have great customer service - very responsive. Even though we've had no major complaints, personally, I have found their system to be a bit "clunky" and takes extra time to navigate.
Our company has decided to bring on a talent management platform (TMP) called Cornerstone (covering an ATS, performance reviews/goal setting, an LMS and succession planning tools). Since we see the value of integrating all those components together, we are dropping iCIMS and moving to the ATS with Cornerstone. Our preview of the Cornerstone ATS seems to have some advantages to iCIMS. A new feature is that candidates can apply without creating a profile. Hopefully the application process will be simpler and quicker!
We are using Hire Centric which is a product of ExactHire. Here is the website if you want to check them out. We are happy with them and just implemented their onboarding program.