I've used it.  You roam on Movistar with T-Mo prepaid, as opposed to Telcel
(a larger carrier with more coverage) on T-Mo contract service.  Either
works fine in Cancun.

It's $1.49/minute, incoming, outgoing, local or to the US, same as on
contract.

I have an old "Gold" account with a high balance I don't use anymore, so I
used that in Mexico instead of my contract service, figuring I was better
off "spending" money I'd already spent on my prepaid account rather than
incur new charges on my contract phone.

BIG WARNING!!!!: Any calls that roll to voicemail while you roam get charged
one minute of roaming- so factor that into what you expect to spend.

be

Either Telcel or Movistar local SIMs might be a better option.  They both
offer a "per call" option where you can call the US for 20 or 30 minutes for
11 pesos (about a $1.00) from your home area.  Depends on how much you'll
use the phone- I was on vacation, so a few calls at $1.49 was cheaper and
easier than buying a local SIM for $30 to get a cheaper per minute rate.
Besides, last time I was there, my hotel (Ambiance Villas at Kin Ha) had
free wifi in the lobby so I made most of my calls there using VoIP (my
Windows Mobile phone has a Voip client so I can switch between cell service
and VoIP at will) at $0.02/minute.

If you want a US number with cheaper airtime, you might consider AT&T
GoPhone temporarily- they claim to work in Mexico now, roaming under Telcel
at $0.99 minute.  You can get an AT&T prepaid SIM with $10 starter airtime
in any retail packaged GoPhone.

Last suggestion- consider texting whenever possible.  It's $0.35 outgoing,
$0.05 incoming, which beats $1.49/minute.

In adition to what Todd Said, all of which is good info, Ive found that EVEN
IF YOUR PHONE IS OFF, you can be charged for forwarding to voice mail at the
foreign rate if you've turned the phone on in a foreign country within the
previous 24 hours. In fact, I've always seen the charge be at least 2
minutes - one for the call from the US to ring the phone and one minute for
forwarding  back to voicemail from the foreign country. TMo says this
normal.
To counter this problem, I set my forwarding to "immediate" so that all
calls go directly to voicemail. Then, at my convenience, I check the
messages and return the ones that need to be taken care of. I tell people
who really may need to reach me to send a text if they want me to call them.
However, if I am in one place for more than a few days, I'll usually just
get a local pre-paid SIM and enjoy the lower rates.