There is one teensy, but major caveat.
If you have a contact that has an iPhone and you message them it keeps that contact as iMessage. If they then switch to something other then an iPhone and forget to turn off iMessage on their phone before getting rid of it you will have a very hard time sending them texts.
You will receive their texts, but your phone will try to send an iMessage to then and fail.
I believe this is actually a fault on Apples server system and not between the 2 phones themselves.
Outdated information not relevant to the OP.
Apple has released a utility to resolve this issue.
This article was updated on Sunday, November 9, 2014 at 3:30 p.m. PT :Apple recently released a new web tool to help iPhone owners deregister phone numbers from iMessage. All you have to do is enter your phone number and Apple will send you a 6-digit code to confirm deregistration. The tool can be accessed by clicking on this link.
Outdated and irrelevant?
And what about the massive amount of people who are not tech savvy and not aware of the situation? The ones that do not know they THEY have to fix the problem.