I'm going to say the ability to play with your friends is obviously a good thing, and depending on how important it is to you what I say next may not matter that much.
But you asked for a cost-benefit analysis, so here goes.
The only Xbox 360 SKU worth paying attention to is the Pro model, which costs $300 American. It comes with a good size hard drive, a headset, and a wireless controller, a pretty good inclusive value.
The real limitation is that it doesn't have wi-fi, so if you have wireless internet, expect to deal with some real pain....the only adapter is made by Microsoft, and it costs $100. If you have wi-fi, I'd suggest against an Xbox 360.
The Xbox 360 has two other major points to consider...the software library and the network.
The software library on the Xbox 360 is very robust. For the current generation of consoles, the Xbox 360 has the most games, with a robust catalog of exclusives relative to other platforms. The limitation is that the Xbox 360 shares a great number of its games with the PC, and more still with the PS3. Given you're on the RO forum, which is a PC exclusive, I'm guessing you have some kind of a gaming PC. Between the Playstation 3 and the PC, there will be very few games for which it will be necessary to have an Xbox 360.
Regarding the network, I will first say that Xbox Live has some of the most robust online service catalogs in the industry. Lots of downloadable movies, TV shows, game demos, etc. Lots of downloadable games to buy.
Next, I would like to talk about how the network operates when you play games. Xbox Live actually requires you to purchase a premium service to even play your games online. This comes in a variety of pay plans, but the only one you're going to want to pay attention to is the $50 per year option. You will be required to pay $50 every year to even play any of your games online on the Xbox 360. Xbox Live integration is supported for every Xbox 360 title, and every game has the exact same core feature set, including achievements, cross game chat/invites, etc. Some games on Xbox Live have dedicated servers, in which case they will be stable, playable, entertaining experiences. Examples include Battlefield Bad Company 2 and Left 4 Dead 2. They are mostly third party titles and are VERY few and far between. There's literally like less than a half-dozen titles on the Xbox 360 with dedicated servers. Every other game, even those published by Microsoft, will operate differently. The network model that Xbox Live uses essentially requires an Xbox 360 user to be the host, which means that he will act as a client and a server simultaneously. This means tremendously bad performance on average. You will experience medium to high latency almost always, and Xbox Live games usually have player counts not exceeding 16 players. Lag will be sporadic, but noticeable more frequent than on dedicated servers. If you've ever played games like Call of Duty 4, Metal Gear Online, or Uncharted 2 on Playstation 3, this is exactly the same thing you can expect with nearly every game on Xbox Live. Alternately, games like Resistance 1 and 2, Killzone 2, and Warhawk on PS3 use the superior hosting method of dedicated servers to deliver their online gaming. Nearly all Xbox 360 games use the same matchmaking and a listen server network model that you find in the PS3 games I mentioned. In my personal opinion, this results in poor network performance and a poor end user experience and there is precisely nothing about this network that should excite somebody who has played games on the Playstation Network or PC platform.
Moreover, consider that they are requiring you to pay $50 annually for an online gaming service either the exact same or inferior to PC or PS3.
The Xbox 360 also has a closed network, which means that there will be no custom maps and no mods. As a result you will be expected to purchase DLC online like map packs and add-ons to expand the life of your software. Playstation 3 has a virtually identical model, but it will be a noticeably inferior system to what you may have experienced on the PC.
My advice is to do some research on exactly what kinds of games you want on the Xbox 360. Make a list of games that you cannot already purchase on an existing platform. Tally up the cost of Xbox Live as well as the system itself. If you have, prospectively, access to enough software on the Xbox 360 that you wouldn't have already to justify the cost, go for it. Otherwise, don't waste your time/money unless your friends mean that much to you
