Weights are not a gimmick. Do you own a g500? I couldn't have cared less about them initially, but now regularly add them and take them out depending on what I am doing atm.
No, but I do own a G9x, which while that mouse is on the heavy side for its size, its not nearly as heavy as the G500. Again, weights are pointless when the mouse is already too heavy to begin with, I don't know if I stressed that point enough the first time around.
Logitech has long since been making their mice excessively heavy to give users a substantial first impression - at first glance heavy personifies the idea of the mouse being solid, tough, sturdy, etc, it gives the immediate illusion of high build quality, and thus makes users more comfortable about purchasing decision. What people fail to realize when they're browsing for their mouse is that greater weight will mean quicker fatigue. While it is true that more weight can mean more stability which can be good for fine tune control of a mouse under high sensitivity, however there's no need for the mouse to be a heavyweight champion
before adding any extra weights.
No matter how anyone spins it, weights are mostly gimmick, not really much more than a toy to play with. Its mostly a feature to help people feel more secure in their decision to spend more than $30 on their mouse.
Also, yes, laser mice used to have real problems depending on the surface. But current models are generally immune to the problem, and anyway, who uses a mouse on a glass table without a mouse pad? A serious gamer/user has a precision pad at the best, or even a common mouse pad at the least.
You're thinking of first generation lasers. The newest generation lasers have problems of their own. Lasers like the Avago 9500s used in mice like the G500/G9x/Xai have inherent positive acceleration around the order of 5%, this problem can be mitigated by using a hard and dark surface, but people who prefer cloth pads can be SOL.
Mice that use the Philips Twin Eye (all Razer laser based mice, the Cyborg R.A.T. series, etc) can have it even worse with jitter on cloth and pixel skipping when lifting off (all surfaces).
I think you're also kind of downplaying the differences. The g500 gets 5700 dpi vs. the mx518's 1800. That's a significant increase in precision.
Higher DPI gives you no more precision if the game engine isn't built for it, and considering developers develop for 400-800dpi mice, 1800 is generally more than enough.
My primary mouse is a Steel Series Xai which can go up to 5000DPI, I have it set to use 450DPI as my primary DPI and 1800DPI as my secondary.
More DPI than you need is useless, you only need "enough".
Additionally, g500 gets 30g of acceleration vs 15g, and allows for 12MP/s processing, which is significantly greater than an optical.
Since you seem to have bought the DPI marketing hook-line-and-sinker I'm going to assume you don't really understand what these numbers mean either.
While its true that newest generation sensors can sustain functionality under faster motion, you only ever see such use with the lowest of low sensitivities, something most gamers don't gravitate towards 5000+ DPI mice for. Some of the most demanding mouse users with low sensitivity use some of the oldest optical mice such as the Intelimouse Optical 1.1 and Explorer 3.0. If they don't need greater than 15g, nobody does.
The processing spec is greater because of the other increased stats, particularly the higher DPI. Simply put, the optical mice don't need faster processing, it would do nothing for the mouse or for the user.
BTW, I'm not saying the G500 is your only or even best option. I just like logitech products as they haven't disappointed me in the past. The G500 is no exception.
And I'm not saying don't go Logitech, I'm just trying to tell the truth and eliminate fud.
Heck, its ultimately hard to recommend against them because they're generally amongst the best values in price/performance/features, and they're one of the few companies who's interface software/driver actually works like it should, and most importantly because they have one of the best warranties and costumer services around, if anything negative happens to your mouse under its 3 year warranty, they'll replace the mouse completely at their cost, they'll even upgrade it if such a change occurred (I know quite a few G9 users who got upgraded to a G9x)