Try something like the Dell multi touch monitor with Windows 7 and some of the free touch pack from MS or the Worldwide Telescope, all the functionality of Surface at a reasonably low cost.  Damn good fun too

What do we mean by successful? Well known in its field, a million downloads, top of the pile in Google searches?
There are many definitions of successful.
I like to think I produce a range of successful software, and altogether I  can identify about 250,000 downloads, but this is small beer compared to something,like Opera, yet search for something generic such as web browser in Google and opera ranks as the 5th entry, a multi-billion pound / dollar budget is not all that helpful either, try typing office and Microsoft can only reach number 5.
Both of these examples are leaders in their respective fields and are considered successful, but does number 5 on Google constitute meeting criteria number 7, either for free or commercial software.
Now try a search for assistive software and check the entries at number 1 and 2, free assistive software, look at entry number 3.  There are over 500 sites and blogs which refer to this site, so is this site more successful than both Opera and Microsoft Office, or maybe its just the fact that this site is aimed at a smaller specialist group of users.
Ultimately, what I am trying to say is that success is something which cannot be defined in such constrained parameters, but needs to take account of things like market sector, competing software relevance, if any.

Taking your original points,

1. I do show some of the software to people, but mostly its word of mouth / blogger reviews which encourages the use, and yes due to the relevance of the software, users continue using it.
2, 3 & 4. Ditto above ref blogger reviews.
5.  Probably 1 request per thousand downloads
6.  Depends, if its stable and fit for purpose, why follow the bloating of commercial apps if there is no need for it.
7.  Damn good position due to reviews and blogs, plus relevant keyword planning
8.  After 250000 downloads and inclusion on many AT freeware compilations throughout the world, not one offer of support or donation has been received.
9.  Difficult, as I plan to get required functionality without extension - see comment about bloating above.
10. I am never surprised as to the uses, but that may be due to the target market and the ingenuity required therein.

So, my question is, is my software successful?

Guys, I demo DNS for clients in situations similar to those barney describes on a day to day basis as part of my work.  DNS has been pretty hot stuff since V10, and if you have good vocal control you can get away without doing any initial training if you want, just set up your headset and away you go. 
To answer CodeTRUCKER, VR in Vista is pants, however it is pretty damn good in Win 7, but it has limitations on voice control and software compatibility which can be overcome more easily with DNS. 
The extra bonus with DNS is the macros are written using a subset of VBA, so you can really make it sit up and sing if you want to.  Another advantage to DNS is you can alter a large number of the built in commands to more memorable words and phrases if you want. 
OS VR does not give you macros or the option to alter the commands.
Hope this helps.