I've been using one for awhile it has the 3 blade kit. So far it seems to be just fine. The blades themselves rust pretty easily but the blade holders seem to be just fine. I do taken them and rinse them though after.
I've started using these Olfa knives and replacement blades. The blades rust - the blade holder / knife seems to hold up pretty well so far. And FWIW - I am really lazy and only rinse them every 2nd or 3rd time they get wet.
They work really great for getting soft corals off just about anything. I can slice Xenia off the back wall of one of our tanks and it pretty much comes off completely clean.
Scrape bubble algae off frag plugs? Yep - works great. Also very easy to get individual zoas off rock / plugs etc.
I've got this kit and I love the different blades it comes with. Also I dont feel like I am going to slice my hand open changing the blades on the ones like anit77 posted. (no offense)