My DD is 2yrs 8 months and to date has been wholly cared for at home by me (2 days a week), DH (2 days a week) and my mum (1 day a week). She will be starting pre-school in April for 2 afternoon sessions. I think she has a great time at home with us - we go to toddler groups, round other people's houses to play, to the supermarket, library, swimming pool, walks in woods, nature reserves, parks, playgrounds, museums, to the shops, mess around at home, DD helps with all the usual domestic chores, we go to soft play etc etc. We chose to work part time and take the financial hit because we want (and are lucky enough to be able to afford, more or less) to be very hands on parents.
But I think she will really really enjoy the fun things they do at pre-school. It is only for 6 hours a week, which I think is right at her age. The rest of the time she will continue to be with me or DH (as my mum will get a year off while I am on mat leave!).
Also DC2 is due in May, and I think it will be really nice for DD to have her special thing - pre-school - when the rest of the time I will no doubt be having to attend to the baby's needs a lot of the time. And of course it will be nice for me to spend some time just with the baby. However "good" our home environment, pre-school will provide something which even the best home environments can't. I am not at all worried about the "academic" side - I am perfectly capable of introducing her to shapes / numbers / letters / colours etc - but it seems to me that she can't fail to thrive by being exposed to a different environment in small doses.
However, since she hasn't actually started, all this is postulation!