We took DS7 and DD5 in December of last year. We go to DL every year, and this was the first year we decided to add on a day at
Legoland. We did it on the first day of vacation, and spent the following 3 days at DL.
We LOVED Legoland. It happened to be a beautiful day (which always helps, of course) and it was not crowded at all. It was clean, and our lunch (hot dogs) was nothing special, but not gross either. We had the coupons where we bought tickets for myself and DH, and the kids got in for free (many thanks to grumpyvet....a dis board poster who sent us the coupons).
Anyway, my point is that if we had bad weather, big crowds, and had paid full price (or even close to full price) the day would have probably felt much different. Maybe going to DL first and then to Legoland after....a visitor might have expectations that are too high. DL can be a hard act to follow. Legoland definitely isn't DL, but we loved it for what it did have, and had a really good time. We liked that we didn't have to split up at all during the day, and could do everything together (my youngest made almost all the height requirements). It was a lovely day, and we plan to include a day there at the end of this year during our annual DL vacation.
Good luck with your decision, and have fun whatever you decide!