Some additional thoughts...

This lack of features issue when roaming used to drive me nuts.  I had a
GSM/analog (Nokia 5190 with the sleeve) phone first, then TDMA/analog
(several, mainly Nokia 8260).  Both had completely seemless roaming.
Literally every feature worked when roaming same as the home area.  The only
time I had seen feature discrepency was when I was roaming in analog mode.
Anyway, I assumed that as long as I had digital singnal, even when roaming,
that features would work period.

Anyway, got with Verizon and discovered virtually all of my "extended"
features fail when I roam in my home state regardless of digitial signal or
analog.  This was a bit of a shock at first.  For some time Sprint was
preferred roaming partner, and didn't even deliver caller ID information
although that specific problem was fixed about 1 year into them being
preferred.  While I continue to lack message waiting indication, I do at
least receive caller ID.  i.e. If I see I missed a call, I can at least
check my voicemail to see if one was left.

Anyway, over the past few years with Verizon I've found their quality of
service where they have service to be very good.

The "other factors" that favor Verizon are:
- Quality of coverage where they have coverage has been very good for my
travel patterns. Mostly I use my phone in my home town of Eugene, where
Verizon arguably has the best coverage of any competitor at least in-town.
I've also witnessed improvements (new towers) more so than with the carriers
I used prior to them.
- I very rarely receive any type of capacity issue. My AT&T phone would
routinely get the all circuits busy during peak times (this is still an
issue for Cingular here in Eugene and Portland too, and it seems as if the
AT&T/Cingular network has seen zero improvement for years).
- Price and feature balance.  They don't have the best geographic coverage
in my state, however, they come close to the best with included roaming
partners, and as you note, you can actually place and receive calls when
roaming-- which really is the most important bit. While it's technically
possible for a Verizon AC user to roam in Oregon, it's highly unlikely at
this point as virtually all available roaming partners are included as
extended network.
- You can tether your cell phone to your laptop, and use data for the cost
of minutes.  See http://cell.uoregon.edu/verizon/data/ for more information.
- Basically, they seem to have a very high quality of service.
- I will admit that it seems like virtually everyone else has a better, and
lower-cost phone selection.  In some cases the competition has more minutes
for the same money, or cool options that Verizon does not.  CDMA phones
(even all digital handsets) tend to be larger on average than their GSM
counterparts and get worse battery life.
- Verizon has the best customer service of any of the carriers I've used
hands down.  I've never waited more than a minute or two to talk to someone.
While I have had to work at times to get issues resolved, they actually have
come through for me repeatedly.
- I hope they continue with the quality of service they have now.