I haven't read any replies, but I'll throw out my random thoughts. DS1 is 15. DD is 5. I also have a 3 year old boy. (This wasn't my plan at all, but reproduction has thrown me a lot of curve balls.)
So...the good:
1) I've never seen a child as happy as ds1 when his little sister arrived. It lit up his whole life. I don't know if the same would apply to you, because dd was his first sibling, and he'd wanted one for a long, long time.
2) I've noticed that both ds1 and a friend of his who also has much younger siblings are more grounded and responsible in some ways than most of their friends. There's an extra level of responsibility and consideration that's absolutely required when there are very young children in the house (even little things, such as being absolutely certain you've chained the door when you leave and things like that, create a subtly different attitude). Both these boys are more...hmm...careful, I think is the word, than most of their friends...careful about other people, I mean. I honestly think that having much younger siblings is one of the best things that ever happened to ds1 (he may not always agree).
3) The younger kids seem to keep ds1 more pulled into the family than many of his friends. If you've read Hold Onto your Kids, you'll get what I mean. DD and ds2 are the most effective means of keeping ds1 close that I have. His younger siblings are just so...cute...that he can't resist them even when he's in a full-blown fit of adolescent moodiness and angst.
4) DD and ds2 adore their older brother. They both light up like a Christmas tree when he gets home from school. They've got one more loving "adult" in their lives than they would if we didn't have him.
5) From our perspective, it is nice to have a "built-in babysitter". We try not to abuse the privilege, though. DS1 often has the choice of watching the kids while I run a quick errand or running the errand for me, instead. We also pay him, if he's babysitting while we're out for recreational purposes. (He gets paid if we're at a concert. He doesn't get paid if I have to drive dh somewhere for work, as we consider that part of pulling together as a family.) School gets out earlier than work does, so it's nice to have ds1 home for 10 minutes so I can run out and grab something I forgot for making supper or whatever.
6) My younger kids actually seem to be having a positive effect on ds1's friends, as well. This isn't really relevant to my family, but it's interesting. The teens can relax the "I'm too cool" thing when the little ones are around (this did take a while in some cases) and just enjoy them. I'm finding the interaction between my little ones and ds1's circle of friends absolutely fascinating. On several occasions, when I'm out with the little ones, I've had a teenager come up to me to say "hi"...but mostly to say it to dd and ds2. Most of the teens really enjoy having the little ones around.
The bad:
1) DS1 is a teenager. Sometimes, he wants/needs to be left in peace, to do his homework, or just to...be. That's really, really hard to get through the heads of little kids who love him to bits and want to be with him. He gets frustrated when they won't leave him alone, and sometimes they're hurt by his unwillingness to spend time with them. DD, in particular, has a rough time with this. DS1 also has a girlfriend, and dd doesn't like that at all....she likes the girl, but not the relationship. (DS2, otoh, is trying to move in on ds1's gf. It's cute, and kind of funny, but it can drive ds1 crazy!)
2) There are times when juggling the scheduling needs of a teenager and two young ones is really hard. Attending an elementary school graduation or high school awards ceremony with two very small children running around is not my idea of a good time, and dh often misses out on these things to cope with the kids. It's easy to get caught up in the needs of the young ones, because they require more care, and overlook ds1's emotional needs. This has made him more self-reliant in some ways, but it is too easy to miss things, and forget about things going on at school while being interrupted by small children demanding food or having a fight or...whatever.
3) DS2 gets into ds1's stuff, and ds1 hates that. He has a lock on his door, and it's his responsibility to lock it, but he does forget sometimes. It's really hard to get a 3 year old to understand that the cool toys in his older brother's room are off limits. It's not a huge issue, but we do have problems with it occasionally.
4) I'm dreading it when ds1 moves out. I'm hoping dd and ds2 will be old enough to handle it, but I don't know. DD gets almost hysterical at the very idea sometimes. It's hard on her to know that ds1 will move out and become an adult and leave her.
I never wanted a big gap between my kids, and this is not the composition of my family that I was anticipating. It has some interesting challenges involved. But...I like it. It's kind of fun, in a weird way. It's meant more responsibility for ds1 and he didn't ask for that...but it's also meant that he's had more freedom to act like a kid without worrying about negative peer pressure. It's been really great.
ETA: I skimmed a couple other replies, and wanted to add:
Board games - We play "kiddie" games first on family game night. After the little ones go to bed, ds1 and I will play a game of cards or Battleship or something. He puts up with Candyland, Trouble or Chutes and Ladders...and enjoys the other games we play (I Spy, Animal Charades, a hunt for poker chipes, etc.).
Outings - Oh, yeah - can be a major PITA. I hate that so many places don't include a teenager in their family pricing. It's frustrating when ds1 really wants to go to the "Krazy!" display at the Art Gallery (comics, videogames, cartoons, etc.) and we end up spending a small fortune and then feeling rushed through the whole thing because the kids don't like it. We cope with it, but it is frustrating and it can be very difficult to find outings that suit everyone. Camping rocks - the kids play and have fun, and ds1 has a great time chopping wood, learning to set up camp, etc.
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