I am sincerely interested in what bad you've heard about the XM-18. You're the first I have ever come across to say this.
I'm a huge fan of the XM-18, but here are some potential issues people might have:
-Flipper requires some wrist action. Many folks are getting used to simply opening flipper knives easily with just the flipper. The XM-18 requires more effort than say, a Kershaw JYD, Zing, etc.
-Finger choil either too small or too large, depending on how you look at it. It can be too small if one wants to use it as the finger can get pretty close to the edge if on isn't paying attention. If one chooses not to use the choil, then it's much bigger than a regular choil (not referring to "finger" choils).
-Thick grind
-Weight
-There have been comments in the past about the stop pin (in the closed position) coming loose. It did for one of mine. But that has been resolved with loctite in all of the newer models. Even in the older models, it really wasn't much of an issue. I just tightened mine and I never had to do it again. With my current non-flipper with the stop pin that has loctite on it, I've never had it come loose.
-Flipper sticks out too far in the closed position. I have a problem with my flipper XM-18 scratching whatever was in that pocket. I liked to keep my cell in the same pocket and it would get scratched up. I couldn't switch that with my wallet because it would be too hard to access the wallet with the knife in the way.
None of these things bother me too much, except maybe the choil on the flipper models. The choil size on the flipper models is even smaller than the non-flipper models because of the flipper, making it one big non-finger choil. However, if you make the choil smaller, you'll have to change the position of the stop pin in the closed position. It's just one of those features that regardless of how big or small it is, someone's going to prefer it to be different. OTOH, you can get extra reassurance that the knife isn't going anywhere with the flipper. I personally prefer the non-flippers.