I have been throttled in the past by AT&T. I have the unlimited data plan. It was pretty rough, because this was back when you hit the cap and they just sent you back to EDGE from LTE. Now for some context:
I used to work in Emergency Management and spend a lot of time on the road. IT equipment can be hard to manage in "newer" departments of the government. My organization has only been around for 10 years (infant in government years). As a result, we never had enough bandwidth to get our jobs done to the satisfaction of our "parent" organization. They sat in their offices that they worked in every day with a bunch of fancy fiber connections fueling the building, and didn't understand why they couldn't Skype with us in the field. Meanwhile, we slept in the rain/mud/fire/desert and worked 16-20 hour shifts. Being the retarded government workers we all are, none of us got along (we're just lazy, they're just stupid, each side sticks to their story). We would hold out for a while, until some of our employees started getting threatened for their jobs (since it's easier to threaten normal people than leaders), then all of us managers would pull out our phones and personal hotspots to augment the government equipment to satisfy our bosses (who denied us satellite time to help alleviate this problem because it costs too much). Being the government, our bosses loved MASSIVE PPT FILES OF DOOM with huge pictures, maps with moving icons, movies, and sound files that must be approved by everyone and their Mother-in-Law with CC'd to third cousins prior to submitting twice a day for the regional boss to ignore the actual content of.
Throw one of these events in a month where I already spent 4-8 days driving around the state of California using Pandora, and I hit my cap.
Now, to AT&T's credit they appear to have learned a lesson from Verizon. When you hit the soft-cap, it sounds like you just get throttled at peak times to maintain bandwidth for everyone that didn't run their data plan for all it's worth during the first 2-3 weeks of the period. This is MUCH preferable to being relegated to the EDGE network, even at 4AM. I'm still a fan of someone enforcing better advertising language to clear this up, but until then it's "Buyer Beware" I guess.
Off-topic, but been brought up by others, is AT&T's call dropping. I have found it to be utterly dependent upon your handset. Android, your screwed. I used to not get any calls until 10PM when all my voicemails would show up on my phone of people wondering why I won't pick up. Iphone, did fairly well but will still drop calls occasionally. Windows phone, pretty good, just clarity issues on my 920. Only drops them on weak network connections. My Blackberry, has only dropped ONE CALL EVER in the 3 years I've had it. I remember vividly because the phone battery lasted over a week when not in use, could make calls in weird places on networks I'd never heard of (I think the "o" stands for original). The one time it dropped a call on me, I stared at the phone in disbelief. My coworkers thought my wife had been killed. I love that phone, and will never let it go as long as it works.