I do most of my birding alone, but ...
Sometimes my wife and I go birding, but after an hour or so she is usually ready to "call it a day".
At other times I bird with my best friend, sometimes several friends, sometimes I take a person around to show them my birding venues, and sometimes I take several people to my birding areas. At other times I bird with larger groups of 10 -15 or so people of varying birding skills.
Last year I agreed to take a "small" group from Kansas City, Missouri, around the oxbows lakes, south of Saint Joseph, MO. I was surprised to find that there were about 40 people who showed up for the "tour"! I had a few misgivings about so large a group, but it all worked out pretty well.
I have not hired an expert/guide for a birding excursion, but I have gone on birding tours with organized groups. Most noteable of these being to Costa Rica, Kenya and Ecuador (these were all done with Elderhostel). (* This organization does all kinds of international tours. I have just done some of the birding ones.) The international tours have leaders/drivers/guides, etc. Most of the personnel, I have found, were quite good at their jobs. I only encountered one guide who was really not up to par. It appeared to me that he would have rather been somewhere else than where he was, leading the group. Sometimes we would go a half a day and not even know where he was or what he was doing.
So, for me, all of the above have something to offer! I suppose it does come down to an individual's preference as to how one wants to pursue their birding interests, and what opportunities are currently available to them.