I sent my 4-year-old to nursery school when he was 2-years-old and he had a great experience there.
Pros:
-social development for your child--where we live there aren't really any small children and my son is very social and gets bored easily so for him it was like fun and games every day
-exposure to different types of children--in my son's nursery school there was a program for disabled children as well and I thought it was a great experience for my son to be around children of different backgrounds and ability level--taught him a lot of great empathy skills, actually
-gets exposed to lots of illnesses early which builds his immune system so when he starts kindergarten (assuming you'll send him to kindergarten) he won't get sick as often--my son very rarely gets ill even when kids in his school are coming down with chicken pox and hand/mouth disease
-helps set a nice schedule for the family which I found helped with a lot of things like 1) potty training (my son was potty trained at 2-years-old) 2) nighttime sleep (he had such great full days that he slept like a rock for 12 hours at night which meant we all got fantastic sleep) 3) naptime sleep (again, my son took great naps in the afternoon after school) 4) appetite and eating (because he was having snacks at school he was exposed to different healthy foods in a social context which kind of gave him peer pressure to eat his veggies and fruits because the other kids were doing it and it was "cool")
-academic development--this wasn't our top priority but it is still pretty cool that by 2.5 my son knew his alphabet and could write some simple characters
Cons:
-gets exposed to lots of illnesses early so most definitely will be ill semi-frequently--for us, this meant that our son got sick probably about once a month where he needed to be kept at home for 1-2 days to recover--nothing major--just a fever, cough, runny nose or maybe once in awhile tummy problems
-cost--it wasn't really a big deal for us because of the type of school my son attended but this might be a factor
-having to arrange your schedule around schooling--again, this wasn't an issue for us but I can see this might be a problem
-high expectations of the school/pressure--never had this problem either because we purposely chose a school that was laid-back and low-stress
-not having your child at home all day--for us wasn't a problem as my son has been quite the traveler since he was an infant--he was staying over at his grandparents' house overnight when he was quite small and we've always held the opinion that a little break from each other on a regular basis was healthy for the whole family--including him--for us, 3 hours of nursery school didn't seem like a huge gap of time.
For us, nursery school was a good decision. I think my son benefitted from the experience and he's a very happy and well-adjusted kindergartener now who is in love with study and enjoys his school life.