The Mark 23 is a niche weapon. For the price of it you can buy a good 1911, or a USP, or even a HK45C with a few extra odds and ends. There is damn near zero aftermarket support for it, in part because of it's price, and the amount of people who are interested in it is very small, so there is no demand for any company to really make anything for it. It's size also limits it to be carried in any type of concealed role, not that you couldn't though, just that some end up finding something smaller than a 5.87" barrel. The sights(both the front and rear) are small, really small, and there are a ton of companies out there who make sights for the USP, but I haven't seen anybody who has full production going on for any Mark 23 sights. The size is, as was already stated, a pain when firing, and I have large hands too. It won't help either if you fire it barehanded, and, after many rounds of firing, you might come to asking yourself if it would be beneficial to stippling your $2000(what I paid) pistol. That is probably not going to happen.
Now the above had to be said. There are some good points. When I had mine, other people at the range would have their target on the 25 and 50 yard line. My target was on the 75. I could hit it too, not as well as on the 50, or even the 25 yard ranges, but it was still cool nonetheless to be able to fire that far and still hit the silhouette. Grouping it was great, but when I bought mine new, a shop made 1911 at that time would have better accuracy then the Mark 23, at around the same price. Then again it wasn't designed to shoot any bullseye competitions with. The recoil system is good, in that it works. I've never had an issue with it. The O-ring idea is okay, but I have my own doubts on it.
Finally, I think H&K discontinued them. I called them up about it some time ago, and they said they did, but they still make parts for it. The supply chain is still in place for those who do have on. H&K said they could sell them direct (when I called them), or you can go over instead to hkparts.net.
In conclusion, I would say that you make a list of jobs for your handgun, for what you want it to do, not what you want your handgun to do for you.