MOST deli roast beef is low in fat and not a bad choice at all. I know it's low in fat at Subway and other similar delis. The only bad thing is that many lunch meats are high in sodium. Just try to buy the lowest sodium kind you can find. I find that certain brands run lower than others. My grocery store has a low fat, reduced sodium variety of deli meats that I love to buy. MANY foods out there are so high in sodium, so it can be hard to avoid them all!
Red meat in small quantities isn't bad for you. Just like chicken, it is always better the less processed it is. Like other said, the sodium content in 'lunch' meats of any kind is usually high. In addition to lower fat versions, many come in lower sodium as well. If at all possible, buy a roast and make your own. If a whole roast is too much, cut it before cooking and freeze some future use. Also, finding other condiments to replace the mayo can cut down on the fat and calories.
It might not be the absolute healthIEST, but it's definitely not bad either.
In my opinion, you can find a way to incorporate small amounts of just about anything into a healthy diet...and a portion of deli meat is still nutritious enough that it wouldn't really be an "indulgence" food unless you're eating a lot of it.
If you can, try and find a reduced fat version because deli meat can be very high in fat (and calories). Also, deli meats are often super high in sodium, so be sure to enjoy extra water with them when you eat them.