We recently spent what seemed like a huge amount of time mattress shopping.
What I found with IKEA was that the mattress prices were good, but they don't fit on regular beds, and you need to buy their bed frame, which doesn't include the slats that the mattress rests on, and oh, it just seemed very complicated. Now, I may have some of that info wrong. But from what I understood, you just can't bring an IKEA mattress home and put it on your old bed frame. And we weren't prepared to buy an entirely new frame, headboard, slats, supports, legs, etc. And the slats serve as the box spring, so you have to buy extra slats in order to get good support.
We went to several stores. Stores always say if you can find this mattress in a lower price, we'll match it. But it seemed to me that there was something really tricky going on. One store sells a "super deluxe premium back saver" by a particular company, and the other store has the exact same thing, but it's called a "super deluxe premium save your back" and they tell you that they're not the same. So the lower-price thing doesn't apply, even though they're made by the same manufacturer and the design is the same. The salesperson will tell you "oh, the 'back saver' has 450 coils and the 'save your back' has 452 coils, so obviously you can see why ours is a better value and oh, costs $1,000 more. And then they go on and on about layers and coils and wrapped and foam and steel and unwrapped. My head almost fell off a few times.
But at Mattress Firm, we found more education. They helped ask us questions. Who was this for? How much do they weigh? Are they done growing? Is this mattress just to take the kid through high school, then they'll move into a dorm, or do you expect this mattress to stay with this person for years and years? If it's for a person who will be sleeping with someone or something (a partner, or a dog, for example), how big are each of them? Is it important for one person not to be disturbed by the other person (my dh outweighs me by 100 pounds and before we got the new mattress, when he'd turn over I felt like I was in a storm at sea). Does the person have health issues or sleep issues?
We hadn't considered all these things (some, not all). It was good to think about this stuff, especially since we were not going to buy the most plain cheap mattress out there. This was intended to be a long-term purchase.
Other things to consider are: the pillow tops can be really thick and need extra deep sheets. The more premium mattresses also can be extremely thick and require deep sheets, so be sure to ask about the mattress depth, unless you plan to buy all new bedding. Individually wrapped coils provide more stability, as opposed to coils that are linked together (that's those commercials that show someone balancing a glass of wine on the bed while someone else jumps on the bed and the wine doesn't spill. It's because the coils aren't connected). We purchased a very high quality mattress protector that is guaranteed for 10 years to not allow dust mites, sweat, pet fur, spilled liquids, dust, etc through. It was over $100 but it means that mattress will be clean. We rejected all the thermo-coils and the fancy layers of cooling products, and pretty blue gel foams, and moisture absorbing beads and and all that. It was amazing how several stores tried to sell us on the lovely colors inside the mattress. Is there anything less important than the INSIDE color of a mattress?
I think that Serta, Sealy, Denver Mattress, etc, are all pretty interchangeable. What matters is construction types, how it fits the person who's going to sleep on it, and return guarantee. The mattresses that we purchased came with a 100 night no-questions-asked return policy. Our daughter has fibromyalgia as one of her many diagnoses and the Mattress Firm person actually knew about pain issues and told us that our choice would not have been good for dd. He helped guide us to a better mattress for chronic pain that is not from a back injury (and it was less expensive than the one we had chosen, so I know he wasn't trying to con us).
We eventually chose the individually wrapped coils, no pillow top, with a 10 year guarantee against sagging, with a 100 night complete come-back-and-get-it and-get-our-money-back return policy.