I've been spending some time trying to find a virtual surround sound setup I can use primarily for movies and some 5.1 music content that I have. In general I prefer listening to music in stereo, but I have some 5.1 DVD Audio that I sometimes enjoy listening to even with headphones. I do game a little bit, but I care more about immersion than pin-point accuracy. In that respect I care more about sound quality, overall effect, and immersion rather than positional accuracy (for my uses positional accuracy was acceptable on all of them, but probably not for serious gamers). Hopefully my experience is helpful for others.
Razer Surround Free: The most finicky from a software perspective, and it flat out didn't work on several computers I tried it on. The overall effect was OK, but I found it the worst for listening to music. I could take it or leave it for movies. Maybe the paid version where you can customize the HRTF is a bit better. You can try it out for free, so there's not much harm in seeing how it works. Since you can get a Xonar DG/DGX or U3 on sale for about the same price as the paid version of Razer, I'd personally go with those options. If you have any Razer products, I think you can get the pro version for free so you should at least check.
Realtek with Dolby Headphone: I tried this with modded Realtek drivers on my motherboard analog out. For my chipset, I do not have the option to select which "room" to use (e.g. DH1, DH2, DH3) and the option is only enabled when I use the front panel headphone jack. For configuration you can set the Realtek speaker output to 5.1/7.1, plug in your headphones, and turn on Dolby Home Theater in the speaker options. If you aren't using the headphone jack this enables ProLogic speaker virtualization.
On first listen, Dolby Headphone gives the most noticeable effect. The reverb/echo creates the most sense of space around each virtual speaker and it does something with the EQ that really fills in the sound. You can hear this in most of the game demo videos where the explosion and other bass heavy effects have a lot of "oomph." For music, the EQ also helps with more acoustic songs, but can become overwhelming with more compressed rock songs. The echo though is just overly distracting for any length of time in many applications.
Blaster X Acoustic Engine Software: You can download the "lite" version for free and manually edit config files to setup different profiles. Once you've got profiles configured, it's easy to use and free. Unlike Dolby Headphone, I can output virtual surround sound via the Realtek headphone out or analog speaker out and set the speakers to 5.1 or 7.1 in Windows. As many people have reported a stereo only output device (any SPDIF) will only let you assign the Windows speakers to stereo. However if have 5.1 or 7.1 sound on your motherboard and are OK using analog out then you do get fully functional VSS. I assume X-FI MB3 is the same.
On the whole, I think this provided the best effect with the least impact on tonal balance. When listening to music, I sometimes thought this created a bit of a "hollow" feeling in the middle of the sound-stage and it didn't have the same "oomph" as Dolby Headphone in the mids/lows. Adjusting the EQ helped fill in the sound some, but Dolby is doing something to really bring out the lows. That being said, Creatives VSS was closest in balance to the source material and did not have any of the echo effect of Dolby. For me, this was the best all around virtual surround.
SBX Surround Pro: I tested this out using a Soundlbaster Omni USB sound card. I could not tell any significant differences between this and the Blaster X. It's possible the different surround values work a little different, but to me the overall sound quality and effect was the same.
Dolby Atmos for Headphones (Windows): This only works for me with Windows set to stereo. Since I don't have any Atmos content, I'm think in practice this is using ProLogic to upmix stereo and then virtualize that. I could be wrong on this and maybe Windows is somehow feeding 5.1 or 7.1 sources into the virtualizer prior to the stereo downmix and it is really virtualizing 5.1 content. Unfortunately, my trial expired and the software won't let me purchase even if I want to for some reason and I did not test this against the others. From my recollection this actually worked OK with stereo music and was the best option for music in general. For stereo sources, I'd still rather listen in normal stereo though. Given the complete lack of understanding of what it actually does and all the issues with the roll-out I can't recommend this right now, but it does have potential.
Overall: In terms of which brand of VSS, for movies only I would go with Dolby Headphone. The extra "oomph" it provides is good for that application and more like what you hear in a movie theater. For everything else, I'd go Creative. I don't see the point for Razer given there are inexpensive options of the others. If you only care about positional accuracy for gaming go with whatever HRTF works best with your ears.
If you have a multi-channel on-board sound card and don't have any noise over the analog output, I'd go with either the software only solutions depending on whether you like Dolby or Creative's VSS. For Creative if you don't need an EQ and are OK editing .ini files then go with the BlasterX software (free). If you want the EQ or easier use go with the X-FI MB3.
If you want VSS over a SPDIF connection to then you are either stuck with Razer or need a sound card with that capability. Asus and Creative have a wide variety of internal and external cards with the capability to apply VSS to the SPIF output in the $30 - $100 range. I picked up a Creative Omni, but I may return it for a more portable solution (Xonar U3 or Creative G1). The software only is fine for my desktop as I have no problems with analog out of my motherboard and the Omni is not as portable as some of the other solutions and the headphone amplifier is over-powered for most of my portable headphones. I may keep it around since it's the cheapest option for Creative's VSS over SPDIF if I ever need that capability. The Omni as a DAC alone seems to be competitive in terms of sound quality to DACs I've listened to under $150-$200 though.
I did not test out Sennheiser's new hardware solutions as they are much more expensive than the other options.