I have this mattress and I like it a lot, but it is firm (5 out of 6 on the firmness scale). Might be worthwhile noting that I also bought the non memory version and that is much-needed softer (3 out of 6 on silentnight's rating). The memory foam is a layer on top and it does get quite warm to sleep in - good for winter but something to be aware of for summer
Got this with the Silentnight divan a couple weeks ago for little over £300. Yes, it's a huge improvement over a traditional spring mattress but it is quite hard for my liking. Pretty sure this is the one which has won all the Which? awards but I would advise trying it out first - pretty much every bed shop across the country will stock this under various guises - according to Silentnight they are all structurally the same, only the fabric pattern differs.
Added heat!! Got my Silentnight from Tesco using a glitch from UKHD last year, got a nearly 400 pound mattress for 164 quid-ish top bargain