That's been my experience too.
1) It seems like there's a bunch of latency when the phone switches between networks.
2) Sometimes, I've seen the network become completely unreachable and not recover without a reboot.
3) A subset of of 2: Occasionally the cellular driver seems to become unresponsive when it's in the not-connected state. I tried to 'nudge' the driver into a state change by putting it in airplane mode (turn cellular radio off) hoping that I'd be able to take it out of airplane mode and cause the driver to reconnect. However, once it got into the airplane mode state, I wasn't able to turn airplane mode off (turn the radio back on). I had to hard reboot my phone to get a connection again.
This issue has caused me numerous dropped calls and frustration with my data connection. I'm about ready to return my phone.
I've had the Galaxy S3 for 3-4 weeks now. I live in an area that is on the fringe of the 4g network coverage, and my hunch is that when my phone is switching between 4g and 3g I am dropping calls. My reception with this Galaxy S3 phone is much worse than any other Verizon phone I've ever owned, and I've been on their network for many years with many different phones. I had a first generation Galaxy and had no problems with it. Supposedly a Verizon rep is driving out to my area to test a similar Galaxy S3 in the specific locations I've identified where I have dropped call problems. If I'm going to continue experiencing this problem, my hope is Verizon will eventually offer me a replacement to fall back to the same 3g network Iphone that served me very well before upgrading to this Samsung S3.