Some time ago, before L was born, we bought a Christiania cargo trike, which we used for the first time when he was about 5 wks old. How is it working out now he's nearing 5 months?
It has been absolutely invaluable for middling-local trips: to the shops (a mile away), the library (ditto), the breastfeeding drop-in cafe (1.5 mi). All of these are walkable distances but a mile is a long way to go when you're carrying a baby plus books/shopping, and have dodgy knees. It also makes the whole thing quicker, and it's fantastic for putting lots of shopping in.
It's also been great for going to swimming classes (about 3 miles), but for now that's about the furthest we can go.
The main issue is bumpy roads. Understandably, L does not like the bumps (the trike doesn't have suspension, and while his car seat is padded, it must still be a bit surprising to bump around in it), and it turns out that the roads round here are fairly bad. We now take bumps very slowly, and as long as he's in a good mood, he copes with them.
Regardless of the bumps, though, his tolerance is limited. It definitely pays to feed him before setting off, and if he's grumpy to start with or it's late in the day, we're more likely to have problems. I have done several roadside feeds, and walked home, pushing the trike, with L in the sling, a couple of times. (I've also cycled at walking-pace halfway along the Thames Path with a grizzling L in the sling, when he refused point blank to go back in and I got desperate and achey, on the way back from the Royal Festival Hall. I wouldn't recommend this, but I don't think it was seriously unsafe given the speed I was going at.) Having said that, he's also fallen asleep in it a couple of times, and if put into it when already asleep, stays asleep quite reliably.
He prefers to have the hood down (so he can see whoever's riding it), which is fine when it's not raining, and even if only raining a little if I cover his legs with a blanket. It gets pretty hot in there with the hood down, too, which he doesn't like. As a result, we leave the hood down unless it's proper tipping it down.
From the point of view of the adult rider: it is still not a speed machine! And it's moderately hard work. I'd like to get a better saddle, too. You get considerable attention when riding it, but almost all of it has been positive.
If you're considering getting one, bear in mind that you do have to carry the baby when you get out. Not a problem for us as we use slings rather than a pushchair anyway, but I doubt you could get a pushchair in there (certainly not a suitable for < 6 months one). General conclusion: awesome and well worth the money, but we haven't been able to use it quite as much as I hoped in the early months (I was faintly hoping to be able to do the 8 mi down to my parents' house, rather than taking the train, for example, and right now that's not feasible). I am, however, hoping that L will tolerate it for longer as he gets older -- he's certainly getting steadily less bothered by the bumping around already. On the upside, I have a good excuse not to cycle a 35-kg bike + 7kg baby + carseat + misc bits and bobs any further than 3 miles down the road, which is perhaps a good way of building those cycling muscles back up.