my oldest one is 3.5 as well - she's been going to day care since month 3.
on the plus side:
1) she's very comfortable w/ other kids; and she's well adjusted in pretty
much any social situation (even though she's naturally shy and will take
some time to warm up). she knows her social boundary, knows how to protect
her rights, knows how to share, and understands how to behave in structured
environments. Just in the last 3 weeks she was invited to 3 birthday parties
from other kids from school alone - two of them weren't even in her class.
this, in retrospect, was probably the biggest benefit of daycare - b/c IMO
being socially behind could further inhibit the kid's social development -
something that's not necessarily easy to catch up. (unlike learning 1+1,
which doesn't make a difference if the kid learns it at 2 or 4.)
2) she hasn't been sick at all in the last 24 month. i think she missed 1
day of school due to sickness in the last 2 years. most she will get is just
runny nose, which lasts at most a few days.
3) language skills - her new vocabs/expressions sometimes surprises me
on the downside:
1) she got sick quite often during the first year, which was tough on us.
2) her chinese is not as good as we want it to be. even though we speak to
her in chinese at home and she can understand it, her preference is english.
3) picking up bad habits from daycare - this one is not really that much of
an issue to me, b/c just like getting sick, all kids will have to go through
these phases eventually - it's never a question of if, but when.