Here's my $0.02.
Mattresses are a very personal purchase. What someone on here loves, you may hate. Often even a husband and wife will have very different needs in a mattress. Personally, I would not be willing to buy a mattress online without ever having had a chance to try it out first.
If you have a mattress manufacturer in your area, I'd recommend going there rather than buying online without being able to test it out.
Online prices do look attractive compared to the $1000+ prices at the big stores for an iComfort. Mattress prices are notoriously over-inflated, but even a $300 mattress that you don't like is $300 wasted. If you have friends who have a mattress that they bought online, try it out. Otherwise, go to a store.
2011 Rockwood Signature Ultra Lite 8293SS, 12K Equal-i-zer
2016 Ford F-250 XLT SuperCrew, 6.2L, 4x4, 6'9" bed, 3.73
2010 Rockwood Roo 23SS (2012 - 2014)
2009 Ford F-150 XLT SuperCrew, 5.4L, 4x4, 5'6" bed (2012 - 2016)