I brought Em to work with me from when I went back to work FT when he was about 3 months old. Actually, both Dp and I worked FT, and split up the time with him, so he "worked" PT at both of our jobs. It really worked out amazingly well for us!
I work for a non-profit supportive housing agency, where I am one of the services program managers. I have my own (very small) office, and don't do very much direct service - lots of computer work & meetings. Working with him was the best when he was pre-crawling. I had a gymini mat on the floor, and toys & books for him (he liked tummy time and back time) and he spent a LOT of time in the Moby wrap carrier. He pretty much took almost all his naps while being worn (sometimes he'd also nap on my shoulder during meetings - too bad I couldn't snooze, too!).
My co-workers and boss were *very* supportive, in fact my supervisor's office is accross from mine and she LOVED having him around. It helped a lot that Em was a super-cute and happy, contented baby who rarely cried. Also, it was great that D and I split the time each day, so we each had a chunk of non-baby time to get things done if the baby-time was not very productive that day. We definitely were *less* productive with a baby then without, but I really was able to keep doing my job while I had him at work with me.
Once he started scooting/crawling, it got a lot harder - he also got a lot louder (not crying, just vocalizing a LOT) so phone calls got hard to do without lots of explanations. Plus he was getting big enough to push out of the Moby if he wanted to. So - we started using a pack-n-play &/or seat at work and started looking for child care at around 6 months. Then I freaked out and wasn't ready to be apart from him so we put it off again and had a harder 2 months of bringing a mobile baby to work (luckily, he just kept getting cuter and more gregarious, so all our co-workers were totally in love with him at that point). It also helped that we got an Ergo - finally big enough for the back carry!
I'm glad I kept him with me as long as I did (until 8-9 mos) ... but there were down-sides, the biggest one was that toward the end I really felt like it wasn't very good for *him* to be there. He had no set routine, and he started to only be able to nap for 30 min at a time. Once he started child care (nanny share with another family) he began to nap excellently, so I think having routine was really good for him.
Since 9 mos, he's been in child care 4 days/week and my DP dropped to PT and has him Fridays. She often still brings him in to me for an hour or 2 on Fridays to nurse and be with my while she runs an errand or two. In fact, the Fridays when he doesn't come to see me (like today) my boss says "where's Em? I miss him."
I think if you have a supportive work environment, it can be really fabulous to bring your baby to work, especially in the early months and while exclusively bf'ing.