I haven't tried your old mouse but from the picture it doesn't look like the Diamondback, and from my experience using the DB a lot for over a year I can safely say that while the optics are in fact good (there are some more or less noticeable problems depending on this and that), it has the by far most idiotic shape ever created. I am very, very thankful that my right hand did heal after changing from that piece of shit.

I had no problems whatsoever aiming when I had that mouse, it really always did close to exactly what I wanted it to do (beaten only by, from my personal experience of course, the Deathadder), but I cannot recommend that mouse.

OH, AND ONE MORE THING! If you are afraid of having to learn a new shape, forget you ever saw the DB I've swapped around trying a couple of mice and that was by far the mouse that took the longest to adjust to. But normal mice really usually only take a couple of weeks and then you don't think about it anymore unless they absolutely do not suit you.

As I said, the mouse that he had a picture of does not look like a Diamondback IMO, but that is just judging from that picture.

I've had many mice, I never owned an MX5xx myself but I've borrowed some now and then and I think those are comfortable mice, but also too "extreme" shape compared to what I am used to (plain microsoft mouse, 1.1a) and soo heavy. The later models had adjustable weights but I never tried those and the shape still isn't really my thing so I'm not going out of my way to try one.

True like you say, it would of course be easier to go to a DB if the previous mouse was like it, but IMO the shape of the DB really isn't similar to any other mouse I know It's just a bad shape and no, when I "learnt the shape" it still wasn't "nice", it was just good performance (compared to most, but the MX mice were better I guess, not incredibly much) and pain in the hands after not long playing at all and after some months my hand suddenly got ruined overnight with really bad pain 24/7 but like I said after I stopped it took some months and then it was fine again so I'm just never again playing with a mouse that is uncomfortable just because the optics is good, and now there are so many good mice besides the Diamondback (and similar) that you just don't have to anymore. The Deathadder has far, far better shape, not too heavy and also better hardware anyway, so my DB's will be staying in their pile of junk on my computer-related-junk-table until I throw them in the trashcan I guess xD

Anyway, before buying a mouse I would read this and this. Well actually I did, and therefore I have the Deathadder. \:D/

Only suggestion i can make is:

avoid cordless mouses
avoid ESPECIALY the ie 4.0. its the suckiest most random mouse arround. Even in photoshop it tends to miss

there are plenty of other cheap optical and laser mouses you should avoid, most of them have trouble keeping data, so you move them too fast and they overflow, not moving at all, or moving completely random.

Aye for that discussion it's simple; people think a good mouse will make them better but while it doesn't, it has the advantage over a bad mouse that it doesn't make you worse. That's the angle to look at it from. If what you already have doesn't limit you, then an upgrade will not help until you're a better player yourself.

Then again, there is always the possibility to adapt your play and still become better and better without the best stuff, such as ZeRo4 and his ballmouse Though a ballmouse that does SOMETHING (if that be a bit weird at times; as long as the user is used to it and can control it) is better than a bad sensor that simply stops reading or even worse bugs out and flips to the ground/air and shit, kinda hard to "use" that in any way.

-edit-
I suggest you read those esreality pages, really. Some of the mice with really good performance are definitely not expensive. I for one still love my ms wmo 1.1a and at the 10€ it costs you get a lot of "bang for buck" But it depends on shapes and how many buttons and stuff you want of course so you just have to read and look for yourself.