For me, it has been absolutely fantastic. Really fun.
I have worked in exciting careers, and had the opportunity to travel extensively to many parts of the world. I have partied all night in many cities, and done many crazy things. I have worked in science, and experienced the thrill of discovery. None of these activities comes even close to the joy I've experienced in raising my son from birth to 4 years old.
He was so cute as a little baby - I saw him learn how to see, how to operate his fingers and arms, and really felt his happiness when he learned how to smile. As he learned to walk and now starting preschool, it has been fantastic. Reading is a very difficult thing to learn, but he seems to be just picking it up. He likes to play in the snow too. Really silly.
Of course, it hasn't been 100% easy; there were (are?) many sleepless nights, just like college. I remember the first time I slept 6 hours straight when he was about 8 months old; it was crazy. Around 2 years all-night baby care became rare.
So the question is, what becomes of the old life? Working late at the office, heading to the bar, drinking all night, going to fancy restaurants, spur of the moment road trips and weekend travel? I found that while I miss that lifestyle a bunch, I'd rather be spending time with my boy. The local public library has a nice play arearea. This is more fun that anything.
What has seemed to happen with work is that instead of wasting time on the internet and socializing, when I have time to work (I am a student), I am very focused.
My partner and I followed advice similar to what Stu W wrote in his answer; we downsized our apartment to make more time. What if you prefer to work like crazy and own a huge house and all kinds of really expensive crap? I have a friend who did this - when the kid was about 6 week old, they put him in daycare 15 hours a day, and went to work like crazy. The kid after a few years seems normal - they choose a nice(expensive) daycare - and their life wasn't that impacted.
The biggest challenge for us was how having a baby changed the relationship with my partner; instead of a two person relationship, there are 3. Combined with the exhaustion in the first year, there is a huge potential for conflict. This is one reason why they say it is important to be married an in a strong, committed relationship. Because it is natural to want to take part in the raising of your kid, if there is separation it generally leads to 20 years of suffering.