This router is starting to show its age and you could be forgiven for thinking it looks a bit low-spec compared to newer products. However, it does everything that the vast majority of home users will ever need, is fully "wireless N" certified, and the fact that it's an older model, competitively priced in a bundled kit with the USB adapter, makes it an absolute bargain.
Setting it up is easy: you can run a 'wizard' either on a PC or on the router itself via a webpage (you will need a wired ethernet connection from PC to router for the initial setup - an ethernet cable is included in the box). Or if you're feeling more technical it's very easy to work through the manual setup screens, all of which include comprehensive online help and documentation (which is much better than the proper User Manual, see 'Niggles' below). The 'out of the box' settings are perfect for connecting to a cable modem (eg Virgin in UK). If your connection is ADSL (via a phone line) don't forget you will need a separate ADSL modem, this router doesn't have one built-in.
The router's wireless setup is straightforward, although a methodical approach definitely helps. As for the wireless USB adapter, with Windows 7 you DON'T want to use the supplied CD to install this on your PC. Just plug in the adapter, make sure the computer has an internet connection by other means (eg wired or another wireless adapter), and Windows will automatically download and install the correct drivers for you. Easy. The router has "wi-fi protected setup" to simplify (allegedly!) connecting a PC to the router. The documentation is particularly bad in this area; it doesn't even mention the WPS button on the right-hand-side of the router. I found it was just as simple to do the setup manually, at least with Windows 7 which has all the necessary options built-in (all you need to do is enter the network name and security key, the same as you entered on the router itself).
Performance: I initially tested the router using my laptop's built-in wireless "G" connection. Speed varied from over 40Mbps with the laptop on the same table as the router, down to 24Mbps with the router at the other end of the house on a different floor. And that was with both the router and laptop's wireless transmission power set to 'low'. So even in "G" mode this router is fast enough to keep up with broadband speeds of up to 20Mbps. I then tried the supplied "N" adapter. If you want to get the maximum "N" speed you must switch the router to "N only" mode, and not connect any other "G" devices to it as they'll slow it down (I initially forgot to do this, and couldn't understand why the "N" adapter was no faster than "G"!). But once I'd made that simple change, the results were impressive: about 3 times faster than "G" in all locations around the house. This matches independent reviews and tests of this router (don't believe anyone's claims about 300Mbps, no products actually go that fast).
So, overall I'm very happy with this router, particularly given its price. If you have a small home network (wired and/or wireless), a typical broadband connection speed of 10-20Mbps, and reasonable expectations of wireless range, you really can't do any better, and certainly don't need to pay any more.
(And if you were to pay more, what would you choose and why? If you need a large number of wireless computers connecting simultaneously, try the DLink DIR-635. If you need that, plus gigabit LAN (and how many people *really* need that, at home!?), then try the DLink DIR-655. Finally, if you need all that plus dual-band wireless (and again, do you, really?), then it's the Netgear WNDR3700. Most other products look over-priced and under-featured compared to this set.)
Niggles with the DIR-615: It would be useful if there were a physical button to turn wireless on/off (to save energy on occasions when you just want a wired connection), rather than having to go to a config webpage. There are too many unnecessary flashing LEDs on the front, which become annoyingly distracting unless you can hide the router out of sight. The documentation (PDF on the enclosed CD) is unnecessarily convoluted and complicated, and doesn't seem to have kept up with enhancements to the product over its lifetime; my advice is use the wizards and/or the router's built-in online help.