I had my DD when my son was 20 months old, found out I was pregnant a few days before his first birthday.
Positives- They are seriously best friends. They just look at each other and grin, they never want to be apart, my son is almost 2.5 and he's a big brother protector, he always is so concerned with where she is, what she is doing. I remember my son being super clingy at her age now (9 months) but she really isn't like that because he is always playing with her, which allows me to get things done around the house. Actually, it's gotten a lot better since she started crawling though. My son is a great talker and he just talks to her constantly, tells her he loves her, he will do anything for her. The close age difference meant ZERO jealousy for us. She is only 9 months old but I know he doesn't remember life before her, and he just wants her to be happy. He has told me before- "mama, feed Charlee!!" when she's fussy lol. Oh, and I loved taking long pregnancy naps with him during the day lol.
Negatives- The beginning was a hard adjustment for me with my time. I was a SAHM who spent 24/7 with my son and after my DD's birth, I felt like I hardly saw him for the first couple of weeks. DH took off work to help and he would take the toddler, I would take the baby, and I felt like I was never getting time with my son. I remember crying hysterically one day during the first week because I had planned to spend the whole day with DS and I had an unexpected doctors appt and wasn't able to. But it has been great for my son to spend more time with other people and he's much less clingy to me than he used to be. He's still pretty clingy but not like he was, it's been healthy for us.
He's not a very good sharer, and she doesn't care much but I'm constantly telling him to GIVE.HER.A.TOY lol. Just normal kid stuff.
During the pregnancy, it was hard at the beginning and the end. The worst was cooking breakfast for my DS while I had morning sickness. I couldn't NOT cook him breakfast but I was always so blech first thing in the morning. Then at the end, I was fat and tired and he would want me to sit on the ground to play and I couldn't even stand back up haha. I also remember once having morning sickness and throwing up, while my DS stood there staring at me in shock. I would puke, then say "it's okay baby, it's okay!" It was really hard just wanting to lie down and not having the chance until it was his naptime.
The absolute biggest negative for me is sleep issues. My DS is 2.5 and just started sleeping through some nights.
It's not every night though. My DD doesn't STTN yet either at all. Last night, I got DD back down at 2:28 AM. At 2:34 AM, DS woke up. I got him back down, went back to bed and an hour later, DS woke up again. I was getting him back down and no joke, less than 10 minutes later, DD was up again. I wanted to pull my hair out. That was an unusually bad night. I think if my toddler was a good sleeper, though, it would be no big deal.
To summarize, i would do it again in a heartbeat. I think the best gift you can give your child is a sibling. The love they have for each other melts my heart daily. The guilt I felt during my pregnancy and during DD's early weeks is totally gone now because I know giving them each other was the best thing I could have ever done for them. I'm actually trying to decide when to have my third. I'm done at three though lol.