I use it because in my opinion it is the best software to create technic models. You can do more things and it has more parts than LDD, and it is easier, more intuitive and quicker to build than MLCad.
Also its 3D models files are compatible with all LDraw tools(MLCad, LDview, etc).
But unfortunately, Sergio Reano, the creator of this awesome program passed away in October 2014.
But today is not possible download the progran fron its official site(HERE).
Quick guide about how to install lastest SR3D in your PC (Windows 7 o newer)
I have uploaded the lastest free available version(0.9.7.81), you can download it HERE.
SR3D uses a very particular software and windows functions to run in and it is hard to get it running correctly.
Usually when try to install SR3D in your PC the installation finish correctly without any advise.
But when you try to execute the program and error message window appears and you can't use it.
I bought a new PC a few months ago and I spend more than 6 hours to find a solution to get the SR3D work properly. I downloaded the newest drivers for all PC devices, newest DirectX, all Windows updates, trying with older SR3D versions, anything you can imagine and nothing worked.
But finally I found a very easy solution for this problem. Actually I don't know why it works, but it does.
This solution worked for in two differents PC, so I hope it work for you too if you can't get the SR3D run correctly.
1 - Uninstall any version of SR3D and LDraw you have in your PC.
2 - Download the lastest version of LDraw All-in-One which included SR3D Builder, it was the 2013_1_v1 version. You can download it HERE
3 - Instant LDraw software tools including at least MLCad, LDraw and SR3D Builder (0.8.9.16 version), I recomend install all tools because they need just a few MB in your HDD.
4 - When the LDraw installation end open SR3D, and finish the instalation (rebuild part list).
5 - When SR3D finish it must work properly with all its functions, but it is not in the lastest version.
Then you can keep in that version if you want, or upgrade it.
6 - To upgrade it just close SR3D and install the lastest version (0.9.7.81) normaly. When the instalation finish it should work properly.
Now if you want you can upgrade LDraw if you want to any version you wish.
I hope this short guide can help you.