XenApp is the leather seats and air conditioning on the car that Microsoft Windows Server 2008 R2 gives you. For sure it adds bells and whistles but at a cost. Plus depending on your needs and number of servers you will have, you may not even use such features.
Add to that there are other products on the market that offer a lot of what XenApp does, at a fraction of the cost. Some names to mention, vWorskpace from Quest/Provision Networks, 2X ApplicationServer from 2X, Ericom and so on.
XenApp adds the ICA protocol to Terminal Services and it is indeed more efficient than RDP and offer more features (like scanner support using TWAIN redirection). 2008 R2 brings a lot to the table too, especially if you are using Vista/Windows 7 as a client (like very efficient graphics remoting, Windows Aero support, etc).
Without knowing your specific needs it is very hard to say if you need it or not but based on my 15+ years of experience with this kind of technology, in 80% of the cases you would NOT need XenApp. If you are talking about a single server or two, I would say it will be more like in the 95% range, unless you have a specific requirement that will need XenApp.
Cláudio Rodrigues
Citrix CTP
