Ok, I thought you had to turn in your phone under Next. At least if you want a new phone every 12 months?
If you want a new phone every 12 months, yes, you either have to turn in your phone or pay it off.
Anyone who has enough usage, or enough lines, to hit the 10GB data package should seriously crunch the numbers for going with NEXT or buying unsubsidized. The $25-per-month-per-line discount makes NEXT/purchased-outright cheaper for my 2 line setup than getting 2 subsidized phones when upgrading every other year as I do now.
I think we'll probably give up our grandfathered unlimited plans, accept the 10GB limit, and enjoy the tethering we don't have now and take the slightly cheaper service (plus save $80 in activation fees).
The key is hitting that $25/month discount level, otherwise the NEXT plans are probably more expensive.
In case it's not clear:
$25/line * 24 months = $600 per line
$600/line - $450/line subsidy you would have taken on your current plan = $150 cheaper
$150 / 24 months = $6/month/line
So your MobileShare plan can be up to $6 more per month than your current plan, and you'd still come out ahead. For two lines, it can be up to $12 more per month, and so on. Compared to my current plan I'll come out something like $100 ahead per device over 2 years, including activation fee savings.
Note the above assumes you're keeping the phone 2 years and no trading up every year.