a lot of people think they're lot better and in some ways they are. they don't slip at all. they have some kind of rubber on the bottom that grabs the fabric and holds on so that when you cut you get a straight cut every time. if you move the ruler, the fabric moves with the ruler exactly and stays in place. they're also very resistant to cuts and scratches, the guy really slams the ruler with the rotary blade and the edge stays unmarred. on the negative side, they weigh a lot. and of course they're a lot more money.
the june taylor, or tailor, ones are thinner and do get cut up. they have a tendency to bend sideways unless you hold the rotary cutter absolutely straight up and down and the blade doesn't always fit properly in the slot. but they're cheaper and they weight less.
up until recently, the martinelli ruler had only one slot, so you had to cut, then move, then cut, then move, etc. with the jt one, you could cut many at a time before you moved it. now it's not true. you can cut several with the martinelli, but imo the price is prohibitive for a ruler.