Walnut isn't the fanciest of woods, but does look good. It's generally a chocolate brown, somtimes with a little bit of blonde/light tan, and dark figure. For riser woods with walnut limbs, I like shedua, shedua/bubinga, or Bolivian rosewood/bocote.
You can use any type string you want on a Chek-Mate and not affect the warranty. I prefer Dynaflight '97 myself, have been tinkering with Formula 8125 lately and like it too.
Beauty is in the eye of the beholder. I've had some of the fanciest bows I've ever seen go through here--hate to see them go. I've also had some that weren't real head-turners at first, but the more I looked at them the better I liked them--not real fancy, but very classy looking.
Some of my favorites are cocoblo, bocote, and chechen for risers; beeswing figure eucalyptus, figured maples, and waterfall bubinga for limb veneer. In a two wood riser I think the best looking ones are a combination of real fancy and subdued woods--like bocote and Bolivian rosewood--less is more to me with these. The rosewood compliments and sets off the bocote. Too much bocote (or any real fancy wood) and you run the risk of looking like a pimp.

I also like contrast--a real dark and real light wood in the riser, real dark limb veneer on the back of the limbs and real light on the belly--kind of a ying/yang or harlequin effect. That's my 2 cents anyhow.
Chad