So, I'm still seeing people getting errors, and as I installed it without any problem showing up during and after the install, here's an in-depth tutorial.
NOTE: I already had Omnisphere 1.5.4 installed, and had no problem during the installation, it actually overwrites the files in STEAM, but if you want to make sure, run the program as admin.
1st Step: Required Downloads
DVDs: Spectrasonics.Omnisphere.2.DVDR.D1.PROPER-R2R and D2 to D8 DYNAMiCS version.
Keygen: Spectrasonics.Omnisphere.v2.0.Patch.and.Keygen.Only-R2R.
2nd Step: Installation
Open the folder named "Step 1 - Installer", then inside that, open the "Windows" folder and double-click on "Omnisphere 2 Installer"
It will ask you to insert the DVD. In order to do so, use an ISO mounting program (I have Daemon Tools)
Mount the D1 ISO release from R2R, when the content from that DVD has been installed, it will ask you for Omnisphere B, mount the D2 ISO release from DYNAMiCS and so on. (C = D3, D = D4, E = D5...)
Wait until the installation ends.
3rd Step: Data Update
Open "Step 2 - Data Updater" folder, then open "Windows" folder, and double-click on "Omnisphere 2 Data Updater". This will be fast.
4th Step: Trilian and Bob Moog patches
In case you have Trilian or Bob Moog Tribute Library installed, open "Step 3 - For Trilian Users" if you have Trilian, "Step 4 - For Moog Tribute Users" if you have Bob Moog Tribute Library.
If you have both, then obviously open up both in numerical order. Go inside "Windows" folder and double-clik on the installer file.
5th Step: Using R2R's tool
Assuming you already downloaded "Spectrasonics.Omnisphere.v2.0.Patch.and.Keygen.Only-R2R", extract it where you want.
Double-click on "Spectrasonics_2048_KeyGen"
You will now see an interface with 2 text boxes and 3 buttons, which are "Patch", "Generate" and "Exit".
Click on "Patch" and point the program to the place you have installed the VST files to. Patch both the 32 bit and the 64 bit version. To do so, select the "Omnisphere.dll" file in both places.
When done, open a DAW (I used FL Studio) in Admin mode, load an instance of Omnisphere 2, it will give you a "Challenge Code". Hit the button "Copy" near the "Challenge Code" text. Paste the code into R2R tool's smaller text box, located under the three buttons. Hit the "Generate" button after this and copy the code it generates in the lower, big text box. Go back to the opened Omnisphere 2 instance and hit the "Paste" button near the "Response Code" text. Then hit Continue.
NOTE: Most of the time, when you paste the "Response Code" into Omnisphere 2, the DAW crashes. I had to do it like 4 times until it worked, so just persist. If it doesn't want to work even after many tries, try another DAW.
If you follow these steps, you should get a perfectly running Omnisphere 2.
If something is missing or unclear, please, let me know and I will correct/add it.