Atomicorp

is anyone using 5.3 in production on multiple servers that have customer sites like phpbb, vbulletin, oscommerce, joomla, etc?
If so, are your customers experiencing any issues - or are they happy or even notice the switch from 5.2.x ?

I've heard of lots of problems moving to this version, and just wondering what you guys are doing before I make a decision.

I'm still with 5.2 branch. I don't expect any problems on my side. but I can say this because I know every single project on my server and can act if something happens; plus I know that all code/cms/... is php 5.3 compatible on my server.
If I would not be in this situation I would make a clone of a server (upgrade PHP and maybe some other major updates) and make some testing like:
setup cloned server
make a new A record like migtest.domaintotest.tld for the test environment available at new IP.
On this environment the customers need to test it and need to approve it.
Of course this scenario would only be my personal way to handle this type of situation I am in.
Other environment, especialy bigge ones, would be handled different by me.

That would be good if this server was mine and the only stuff on it was mine - but with over 100 servers and tens of thousands of domains its a little harder to just update and see if it works - customers are a little less forgiving then we would like

No problems with WordPress and I've been running 5.3.3 for months. Vbulletin should be safe as well. In fact, I'd be surprised if you had any problems with open source software on 5.3, given that it's been out for years. The problems with major versions almost always come down to custom software and something not working right. Keep in mind that most of the major changes will give you a warning (like split() is depreciated) and that's about it

I am using 5.3.6 without any issues both with vBulletin 3 and 4 series. For vBulletin 3 series i had to fix couple small problems in vBulletin which are not anything major. vBulletin will still function correct just add lots of entries to your error_log.