Browser specific websites used to be the norm in the early days of the web. Back in the days when a site would tell you what resolution you should be using and what browser works best. This should be a thing of the past, but every now and then I still see “This site best viewed in such and such a resolution” and sometimes a splash saying “This site is only supported in Internet Explorer”.
Now the first issue of a preferred or best resolution isn’t quite as bad as the later, but really, the user is at your site and probably has no ability or intention of changing their resolution just for you; why bother even mentioning it.
Go home, we don’t want you here
The second issue is more akin to opening up a store and punching every third or forth customer in the face and telling them to go home. There are one of three possibilities, the first one is that the user will obey you and fire up the browser you are demanding they use. The second possibility is that the browser you are forcing on them does not even exist for their operating system. The third possibility is for the user to laugh in your face and move on to the next best site that’s offering something similar to you and never come back to yours.
I’m not saying you should make your site work in every possible browser out there, but don’t purposely block your visitors, you may actually be blocking clients that can render your site just fine. Even if your site does break in their browser I believe a broken site is better than a “Use this browser to view my site” message. If your site breaks in their browser you have either an error on your site or they are using an old browser. If it is the later chances are they are constantly seeing sites break and hopefully will upgrade on their own someday. Either way, they can still get on your site and you haven’t made them feel inferior.