Chalk me up as a user for whom Messages seems to function as advertised. Most people don't take to the air to shout about how something as mundane as text messaging works fine for them.
If I use Messages just on my iPhone, it works just fine. However, when I use it between 2 macs, an iPhone, and an iPad, it just breaks.
Sure, it's an edge case use - most people use messages just on their phone - but it's a pretty basic feature.
Works fine for me across the board. My only issue with it is how it's eating ~2gbs of disk space on my phone for attachments. I haven't figured out a way to get rid of it without going through and purging all my messages or formatting my phone.
The only odd behavior I get is that, occasionally, the desktop client will take a moment to catch up to the other devices, or be missing chunks of a conversation. That usually sorts itself out in under two minutes or so, though, so I assume it has to do with the time it takes iCloud to respond.
EDIT: I'm still on 10.8, if that's relevant. I intentionally lag a point version behind because of audio software (the latest and greatest always breaks audio).
Occasionally there is a hiccup where a conversation thread is not immediately synced with the other devices, but that seems to be happening less and less.
Also Mavericks removed the ability to disable graphical smileys :(((((( (Is there a workaround or fix anyone knows? please tell me!!)
I mostly use Messages on Mac but use iOS some too. There's a huge problem on iOS which is scrolling to the bottom when people type stuff, which prevents you from catching up on conversations until everyone stops talking. That's super frustrating.
(Auto scrolling to the bottom when you talk, on Mac, is also annoying. I want to read through chat I missed, reply to things as I go, and not lose my place. You have to open a second copy of the window to deal with this.)
Hangouts is pretty good too, although a few nitpicks - the emoji icons are ugly as sin (I'm willing to bet that some sort of copyright/patent law is to blame here), and it's gotten a few messages stuck (to be fair I can't say the same thing hasn't happened to iMessage).
Strangely, both Hangouts and iMessage are better than plain SMS though, even with just sending plain text messages back and forth.