Haven't used it, but looks good - 5lbs per cubic foot density foam should be reasonably firm, and 4" thick memory foam should be good (I wouldn't want it that thick if it was a less dense foam - you'd sink in too much).
The price looks about right - $850 for a king. For comparison, you can get 3" 5.5lbs density memory foam on a 5" base for $800 from
http://www.bergad.com/mattress.html (or $999 from
http://www.isoform.com/ which is the same company!) , but shipping might cost you $100.
If you want to save a bit, you can get a mattress kit from Bergad's fabrication site :
http://www.foam-fabrication.com/mattress.html
This is the same 2 foams - you just get them shipped separatly with the cover, and put it together yourself. This is what I went for. The king kit was $480, and it cost me $100 to ship it right across the country. For me, the savings were worth the 15 mins to put it together...
The cover I got with mine said 'isoform medical' on it, and is REALLY nice - but when I ordered it, I was told you got a standard toweling cover with the kit, so I can't be sure I didn't get a better one by mistake - YMMV.