I've been looking for a new mattress from one of the various mattress in a box stores and this seems like a good deal.
50% of all sizes, free delivery and 125 day trial period with free returns. Seems like a low risk option.
These mattress are well reviewed but I find all the mattress in a box retailers are suspiciously 4.5+ stars with hundreds or thousands of reviews.
They also all seem to always have some discount code, which is a bit dodgy but 50% off is the best I've seen.
I have one of these (king), love it. If you've been eyeing off any of the bed in a box brands, get this one purely because of the discount on offer - 50% is the highest I've seen also.