I have used an original Marathon mouse, and I don't mean prior to 2018, I mean I bought this very mouse in 2014 (for 20$ at a Costco no less)Why am I getting a replacement? The dredded double click has only gotten worse these past few months. My mouse has had over 8000 hours of World of Warcraft, gone through high school and my first two years of college, outlived 3 different laptops and has not only been tossed around but traveled to over 6 states on vacations and survived 2 moves. I replaced the batteries in it for only the 3rd time since its purchase and this mouse has been nothing but incredible to me.I just received my "new" one today, and I can tell, this is not my mouse. I've used it for 6 hours in both searching the web and some WoW.With my original being almost a decade old I think I can say that the quality on the original is tenfold of the quality on this new one. The feel is different, the original is smooth and made of a shiny plastic whereas the new one has a matte finish and not as smooth of a surface (Im talking about the top of the mouse, to move your hand in adjustment isn't smooth so I have found myself letting go and then moving fingers rather than just sliding my hand further up on the mouse).The biggest complaint I see from people is the pads on the bottom, but I had a lot of issues with the ones on my original so having them be plastic is a better deal for me. I do not have a mouse pad and have never used one, so the rubbery ones have had to be superglue back in place a few times from rubbing on the desk. You can still pull the new ones off to get to the screws, but I never had luck trying to fix physical issues with my mouse anyway.The scroll wheel is similar, but different. I dont know how to describe it exactly, but I had to download the software to turn on "smooth scrolling" because what is supposed to be a free scrolling wheel would zoom down the page and slowly scroll back up, which I found to be really obnoxious. Similarly, the new mouse was a lot slower out of the package and I had to get the software download to change the cursor speed to near max just to have a comparable speed to what I am used to. So these mice are not even comparable out of the box, as I have never used the Logitech software on my original mouse.The most interesting change to me is that the new mouse is 5 grams lighter. I think this can be attributed to the thumb rest on the new one being a lot thinner and more of just a plastic piece than an actual thumb rest like the original. Not only that, but the majority of weight seems to be at the scroll wheel with the new mouse and at end of the mouse in the original (different center of gravity? I don't know, but it feels like I'm moving it with my fingers rather than moving it with my palm). Either this is due to the factory batteries being smaller in the new versus my replacements in the old or the guts of the mouse have been changed/moved, either way this is a weird change which has thrown me off a little.I honestly never thought they still made this mouse and I would have to try something new (which I have never been all that good at, hence the original one is 8 years old) which is why I have waited so long to find a new mouse. While this is not my mouse and it never will be, it is the closest replacement I can get, and that is why I give it a 4 star.TL;DR:The original is a lot smoother to the touch, has more thumb support, requires no configuration to mouse settings for regular use, and is made of quality plastic.The new version lacks a decent thumb rest, requires configuration to mouse settings in order to be usable, is lighter and has a more central center of gravity, and lacks a thumb button.Both are very responsive, use the same scrolling feature, can be programmed, and have the same shape.I cannot attest to battery life, longevity, or other features as I have used the same original mouse for around 8 years and the new one for only 6 hours.