I really see so many benefits to so many different scenarios--it really is one of those things, in my opinion, where you sort of take the good with the bad and just do what works for your family.
That said, there are 21 months between my 5-year-old and my 3.5-year-old, and 33 months between my 3.5-year-old and my 9-month-old. Here's what I like/don't about those gaps:
My first two are close enough that my oldest didn't seem to have a big "transition" when his little sister was born, but quickly accepted our new family. I was still totally in "baby" mode when I got pregnant, so it was easy for me to transition back to having a newborn (I was still diapering my oldest when my second was born, still nursing, he didn't have tiny-pieced-toys yet, et cetera). By the time my third was born, my then-youngest had been potty trained for a year and our house was full of legos and playmobil. While some people would appreciate the break from diapers/baby-stuff, it made it harder for me to go back to it.
On the other hand, I loved experiencing my third pregnancy with two kids who were old enough to "get it." My oldest was nearly 4 when I got pregnant, and we were able to explain it to him and enjoy showing him pictures of fetal development and things, even before there was any "evidence" of the pregnancy. He was fascinated by "actual size" pictures of the developing embryo. My daughter, who had just turned two when we became pregnant, didn't necessarily get it right away, but by the time I started to show a little--and for SURE when she could feel the baby moving--she understood and enjoyed talking about it. I really enjoyed sharing my pregnancy with both of them--especially my oldest.
The hardest thing about the almost-3-year gap was the difficult time my now-middle child had in assimilating to that new role. The baby is 9 months old now, and I still feel like my 3-year-old is "adjusting." Despite a lot of awareness and deliberate effort on our parts to avoid it, I feel like this transition has been really rough on her, and that's been difficult on both practical and emotional levels.
Actually, as I reflect on my own experience with my own kids, I'd say that I see a lot of advantages to having a small gap--say, 2 years or less--and a lot of advantages to having a larger gap--say, 4-5.5 years--but, for me, I think ~3 years is a tough gap. In my opinion, 3 is a difficult age anyway, and in my experience, compounding that difficulty by introducing a new sibling has been rough. (But of course my experience could have everything to do with personalities and little to do with ages.)