edit: oddly enough, another alternative is to sign up for one of those providers that have free wifi hotspots all across the city (e.g. CableWifi, XfinityWifi), which could be cheaper than a data plan
I certainly don't know enough to say but I imagine someone here might.
When I got home (US) I bought an iPad with AT&T 3G and put that SIM card into the 3GS. I had to install a special profile to make everything work, but that was all it took for a data-only 'phone' plan.
It's a bit klunky. I make calls with an app called Talkatone, and the quality when I'm on 3G gets very bad. It's better at home and work over wifi, but some friends still complain about sound quality and lag.
I switched to T-Mobile prepaid on their 5GB "unlimited" plan, but there are even worse drawbacks there. For one, I struggle to get service. My apartment is half underground, and neither VZW or TMo have coverage, even though I'm in the city. AT&T does, but they don't deserve my money. The second issue is that it only comes with 100 minutes, and I can't figure out how to activate the 10c per minute overage (my phone just tells me I'm out of minutes and I should upgrade the plan).
My SO gets frustrated when I'm out of contact, and is seriously pushing for me to get back on a contract. Maybe Google Voice and a SIP gateway would be a better option? I really don't know.
I find that the BYOP offerings of Net10 and Straight Talk don't differ that much: both offer LTE, similar APN settings, same throttling limits, etc., as both companies are owned by TracFone. The only difference is that Straight Talk costs $45/month while Net10 costs $50/month, although if one sets up recurring payments on Net10's website the cost will be reduced to $45/month as well.
[0]: http://androidforums.com/net10/763501-net10-t-sim-card-data-...
For the past year or so I've been using a $10 plan from Airvoice Wireless (an AT&T MVNO) for voice & text on an unlocked old BlackBerry (= good signal quality + long battery life). I'm not sure how long Airvoice will last (they seem like a tiny operation) but at least their customer service has been more responsive than Net10's.
I do Straight Talk SIM with an iPhone 4 on AT&T's network and the only problem I've had is no media messaging, which is not a huge deal. Also, iMessage is great for that. If you care about MMS, buy a phone from Straight Talk directly. The iPhone on Straight Talk is on the Verizon network, but since it's a VZW phone, you can use SIM as well.
Straight Talk SIM now supports LTE on AT&T if you want to go that route.
The point is, for phone service, you don't need to spend more than $45 a month and off contract phones can be had for $400 or less for top end phones (like iPhone 5 or Galaxy S3). If you don't want data, you can drop down to $25 a month or less.
There is no reason to be on contract, it no longer makes financial sense.