OK, so gating IE 6,7, and 8 is fair. What about IE 9? Throughout your responses to comments you've grouped 9&amp;10 together. Consider the caniuse.com matrix of which modern browsers support which parts of HTML5 and CSS3,

http://caniuse.com/#compare=ie+9,ie+10,firefox+14,chrome+20,safari+5.2,opera+12

Based on that chart, it looks like any developer that has to support IE 9 &amp; 10 will have to do special, different, custom work (e.g. finding and testing shims and polyfills) for IE 9 apart from all other browsers. So, it seems disingenuous to claim that IE 9 is in the same ballpark support-wise. 

If you look at IE 9 compared to the latest mobile browsers, there is more parity; on the other hand, it looks like there is a special flavor of IE9 shipped with Mango whose features aren't reported on caniuse.com.  So, maybe it's not fair (to IE9 Mobile) to compare baseline IE9 with the other mobile browsers.

http://caniuse.com/#compare=ie+9,ios_saf+5,op_mini+5.0-6.0,op_mob+12,android+4

Nevertheless, is it fair to gate IE 9 if a site uses modern HTML5 features such as Web Sockets, HTML5 forms, IndexedDB, or WebGL, or CSS3 features such as gradients, animation, or multiple column layout?

Now here's the thing. IE 10, as great and modern as it looks to be, hasn't been released.  It's still in preview/beta land.  Most sites don't support unreleased browsers.  Most users don't have unreleased browsers.

Consequently, I (and many others) don't understand your position given the facts.  Every released supported version of IE has merely-frustrating-to-crippling support costs which exceed the costs of just supporting the competitors' (auto-updating!) products while the only version of IE that achieves some form of feature parity is in beta.

PayDirt's user-agent sniffing might be a sloppy hack, but their thought process certainly seems sane.