My son just turned 5. We never did any preschool (I was certain this was a disadvantage to him academically), we do a lot of playgroups and we do park district classes, one we picked up just before he turned 5 was a play school class that ran for 2 hour twice a week. That class did wonders for him socially but I don't know how much it helped him academically because he already knew most of the stuff they were teaching. The best thing he got from it was writing practice. He is VERY social.
Despite my feeling like I let him down by not having him in preschool of some sort since he was 2 or 3 like so many other city parents, my kid just got accepted to a really great gifted program for kindergarten. He took both tests (classical and gifted) and scored really well on both, he was offered a seat in our first choice regional gifted center school, which, if you know Chicago, you know this is a really big deal.
I only mention it because I know how it is in the city, I feel like my kid is the only kid we know that didn't go to some sort of preschool program for at least a few days per week.
So, academically, if you're playing at home (I never do "school time" at home, we just play and learn and talk about things), and if you're going to playgroups and taking park district classes (I'm a big fan - cheap and you can find some great ones), preschool isn't an absolute must.
My child will be starting Kindergarten this fall with no more experience than a $100 per session 2 day per week park district play school that he didn't even start until he was almost 5, and not only will he start kindergarten, he got one of a very few number of seats available because his test scores were ridiculous.
If you've got questions about park district classes or anything, you can message M.. It's a hard decision and if you're like M., the hardest part was feeling like you were disadvantaging your child if you didn't put him into a preschool, but at the same time you want control over his environment while he's still young and developing so you can correct problems and encourage good behavior as you see them happen.