We have friends that fostered two sisters several years ago and ended up adopting them. We were all a little surprised, as our friends were only about 26 at the time, and these girls were 6 and 7 years old. The wife had taught one of them in first grade though, so there was that connection. Anyway, things seem to be good for them. One of the girls is well behaved and seems to be very grateful for the life she now has. The other misbehaves often in serious ways and seems pretty ungrateful. She is definitely an attention seeker. However, you never know how your biological kids will act when they're adolescents either. I don't think our friends have any regrets about the fostering and adoption, despite their struggles with the oldest girl. They've had two children of their own since the adoption, so they have a very full house.
My mother in law has a sister who is actually a foster sister. She was never officially adopted, but she is treated as an official member of the family. I think in their case, foster parenting probably saved her life. She grew up in a normal household with brothers and sisters. Her children grew up with grandparents, aunts, uncles, and cousins. I often forget that she's not actually a blood relative.
So, while I have not fostered myself, the people I know who have have had a pretty good experience with it. Good luck and God bless!!