We have a 6 year gap. I was going for 5 years but it took longer to conceive our second.
I like that we got to focus on our older daughter till she was 6 and getting more independent.
I love that I got to focus on my new baby whilst my older child was at school. We went to baby groups and coffee with mums of small babies rather than the baby being taken along to toddler groups. I got to sit on the sofa feeding and watching boxers again.
My older daughter isn't too jealous as she understands a baby has high needs and she got that level of attention when she was a baby. She can pick her up, carry her and almost never drops her! She understands what can be dangerous for babies so keeps her lego on the table or in her room, let's us knowing the baby has got something she shouldn't have etc. If my daughter and the baby both need me my older daughter has the maturity to wait.
What is challenging is their completely different needs. One still has two naps a day whilst the other enjoys full days out. A lot of stuff my older daughter is big enough to attend now isn't suitable for the baby so we sometimes split into two groups of two or three and one at holiday day camp/childminder. There will be two separate places to take them and pick them up from on work days for three years (this could be solved with a childminder but there are none that do school runs to my older daughter's school/a general lack of childminders).
I'm happy with my gap and glad I won't ever have a baby and toddler at the same time, two lots of full-time childcare to pay for at once, two doing exams at once, two at uni at once, even two hormonal teenagers at home at once!