I guess I would say "it depends".
I have had good experiences and not so good experiences diving with unfamiliar buddies.
I have been on liveaboards where I have been paired up with someone I had never dove with before. Generally the experienes have been good.
On trips to Cozumel, especially when travelling by myself, I have been buddied up with divers I had never dove with before. I have had some very good experiences, and made a lot of friends. On one trip to Cozumel I was buddied up with a mother and her 15 year old daughter. The 15 year old was a great diver, even with limited experience. The mother was a very good diver as well.
Locally on a recent dive I met my buddy at the dive location. He told me he had invited a diver he had just met and dove with a couple of days prior.The new diver had moved to the coast a few months prior, and according to my buddy, the other diver claimed to have over 100 dives, and was comfortable in the water. We geared up and head out as a group of three. Part way into the dive my buddy signalled he was getting low on air, was going to head back to shore, and that I would be buddies with the other diver. I checked the other divers air supply, and determined that we could spend a few more minutes exploring before we headed back to shore. Suddenly the other diver was right on my back, trying to hold on. We made a somewhat uncontrolled ascent (I was dumping air out of my suit and BC as fast as I could) from about 50 feet. Once on the surface I made sure the other diver had gained positive buoyancy and had his breathing under control. Once he had calmed down he was able to explain that he had forgot to put on his ankle weights, he had become buoyant in the legs, and was headed for the surface. He had grabbed me from behind trying to stay down. We descended again and made our way to shore. A couple of days later, after relating to my buddy on what had transpired, he politely advised the other diver to take a dry suit course to gain more experience prior to diving with us again.
A couple of years ago, I was asked to take a couple of divers on a tour of a popular dive site. They assured me that they were experienced, and were good on their air supply. About 10 minutes into the dive both signalled they were low on air - if I recall correctly one was down to around 500 psi. I immediately signalled for all of us to ascend. Once on the surface I directed them to surface swim to the exit point. I will not dive with them again.
From now on, I will be more careful on who I dive with, asking more questions to get a better understanding and feeling for their experience.
Divegoose