Hi MeJane,
Jamison certainly makes some good quality mattresses that have better value than most mainstream manufacturers but no matter who makes a mattress you are considering I would always make sure you know all the specifics of what it inside it (see here) so you can compare the materials to the guidelines here and make more meaningful comparisons to other mattresses.
I would also be aware that a mattress warranty has little to do with the useful life of a mattress or when you will need to replace it (see post #174 here) because mattress warranties only cover defects and not the loss of comfort and/or support which is the most likely reason you will need to replace a mattress. Knowing the quality and durability of the materials inside a mattress are always more important than the length of the warranty.
Phoenix