Is it really useful? Ive got full set of DMM Wallnuts, cams from 0.5 to 3 (some doubled up). Would those Tri Cams would be fine addition if Im used to cams already?

I like using them in horizontal cracks where weighting a cam would bend over the stem. Also lighter than cams. But yes they take longer to place than a cam. A couple of them are a good versatility addition rather than adding doubled-up cams.

Although not as useful as an SCLD, Tricams certainly have their place. They are light, strong, cheap and simple, but slow to place and you have to really be careful to get a good placement. For this reason, I love them for multipitch, because I can use them in the anchor and save a cam for when I'm leading. Unless you have a good stance, however, they don't really pull their weight - I don't think people really use them much above 5.10- or so. I'd recommend them to add diversity to your rack, as there are plenty of places where you can only get a tricam to fit. People who climb at the gunks love them for this, as the floppy nylon slings do better than cables in horizontal slots.

Mike, These tri cams are completely different from the cams you already own. They ARE useful, especially the smaller sizes, and are worth looking into. Spend a lot of time practicing with them, as they can be difficult to place and you want to KNOW when to use them. Personally, I wish I had gotten a set of hexes before I invested in any of these guys. They're nifty, but not very versatile.