These days, I run most everything with friends. I have more fun this way. You can share success with friends more than you can with random people that you have never met.
Killing a boss or clearing an instance for the first time is much more fun when you have people you enjoy playing with along.
I also helps in a variety of other ways. The dungeon finding tool was great for Wrath because the instances were just so facerollable. Now they are difficult, and people are bigger assholes than ever. Some of the best reasons to run with friends.
1) Live and Learn: People in WoW are not generally patient. If you are learning new content, it is best to do it with friends. Friends help. Randoms criticize. How many instance have you been in where someone leaves after a single wipe saying how fail the group is? Friends stick to it, and the end result is a greater feeling of accomplishment.
2) No loot drama: A group of friends will not be apt to ninja something from another friends. Someone going in as DPS from the dungeon finder may well snatch an upgrade from a healer of tank. Friends arent going to do that. They will generally look at what is best for the group and pass on it if someone in their main spec needs it. No loot drama makes for a much more relaxed atmosphere.
3) No talking shit: Pugs talk shit. Some of them are good people, but many subscribe to the Greater Internet Dickwad Theory. They don't know any of you, so they just don't give a fuck. Friends are obviously going to be different. If someone makes a mistake, it gets explained and fixes. Not looking down. Not being a douche. People learn and move on instead of getting pissed off.
4) Sharing Success: When you are playing with randoms, you are not invested in those people. When you are with friends, you are. If someone accomplishes something they never have or got a piece of loot they have been trying to get forever, you are genuinely happy for them. It makes your experience that much better!
In conclusion, play with friends if you have them. It will make your play experience much better....unless your friends are douchebags.....then find new ones.