Let me chime in....
Tried AT&T call blocking app for a few weeks and it didn't work that well with me. It didn't block most of my spam. App was deleted after the two weeks and had to call ATT for it to be taken off because it blocked a few 800 numbers that were not spam even after i deleted the app. Very strange.
Just installed Nomorobo app today, will see how that goes but off the bat, it does not have a Today Widget Notification like TrueCaller or Sync.me. Did a few test spam numbers and it didn't say it was spam in the app and i then reported it as spam. This app doesn't have a search phone number and therefore will not tell you where the number is coming from and what it is. App only has a "Report to Nomorobo" feature. I have to spend more time with this app, it does seem to have a simple and clean UI and i do like the this app seems to respect your privacy when it comes to your contacts as it does not request it.
Have had TrueCaller and Sync.me (non paid subscriptions) apps installed running side by side for a few months now. They do work and block i would say 90% of spam. Definitely dig the today widget that you can quickly to a copy of a phone number and the pull down on the today widget and both apps will show you the name and location where the call was made from. This is a very useful feature to me. Apps do require your contacts info and do ads with eh free version. Although i found TrueCaller to have less ads and not as intrusive to the function of the app itself, Sync.me ads take up the whole screen of the iPhone and constantly asks for your o sync contacts from FB, Twitter and Google before every spam filter sync. Both apps allow you to search phone numbers that were not even called in, also a nice feature.
Going to give Nomorobo a few weeks to see how it works but due to not having a widget and inability to search numbers, i think to me its not worth the $20/year subscription.