There are two clearly divided camps when it comes to device detection within responsive web design.
For & against.
Once you remove people from their oversized horses and put the soap boxes back in the bathroom cupboard you are left with the choice of including a server side technique to assist you in providing the user with a more specific website experience.
The reason why some responsive design ninja's object so strongly to this technique is because it's breaking away from the standard rules that responsive design follows: Fluid grid based layout; Responsive Images & Media; and Media Queries. The main area of concern for these ninja's is that they think that because there is device detection then there may be a specific mobile website.
Instead what the device detection should be identifying are the capabilities of the device that is being used to access the website so that the appropriate elements of the site can be returned to the user.
The practice often brings with it a segmentation of the code and the content because it now becomes possible to serve a different user experience. Is that responsive design? Well, not in the true sense of the meaning.
Our main driver when building responsive websites is to provide a single site with a single source of content and codebase that is able to respond to the users device regardless of the size or capabilities of that device. We have some front end detectors (modernizer) that assist in ensuring we either progressively enhance or gracefully degrade the website features to match that of the users device.
If you are leaning toward device detection you should be also thinking about whether or not you want to lean a little further and look at Mobile specific website options in the same way that sites like LinkedIn have done.