I love the bathroom sink when doing infant potty training. Yes, it may sound gross, but it's just oh so handy. I have used the sink much more extensively with my daughter than with my son (boys make more of a mess with the sink, unless you're good at aiming), and we're just now starting to use the sink less often, but here are the top 10 reasons why I love it:
1. My babies like the security of being held in my arms over the sink; when they're relaxed they tend to go more quickly.
2. The mess can be easily rinsed down the drain.
3. The baby isn't taking up a spot on the toilet, so it inconveniences others less.
4. Pee is sterile, and poo isn't really all that germy unless baby is sick or has just had some vaccines, so it's really not all that gross. Especially since you're going to rinse it all down the drain. If you're a germaphobe, you could also spray with disinfectant.
5. It saves your back. You don't have to get up and down off the floor, or otherwise put yourself in an awkward position.
6. Babies just love looking in that bathroom mirror, which again, makes them relax, and helps them "go."
7. If your bathroom sink is in a vanity with some counter space, you have an instant changing table!
8. You're within easy reach (usually) of the switch for the vent. My daughter finds it super relaxing and will calm down if she's fussing, and then she'll pee.
9. You can easily reach the handle to turn the water on. Water sounds are a surefire way to get your baby to "go."
10. My sink ends up staying cleaner, because I'm constantly rinsing it and making sure there's no hint that a baby just went potty there. Thus, my sink also stays free of toothpaste and other messes.
The downside is that once you start solids, the sink becomes a thing of the past. Exclusively breastfed baby poop is extremely liquid (you might worry at first that your baby has diarrhea, but it's perfectly normal), but once you start giving any amount of solids, it will change the consistency of the poop into more of a "soft-serve" type consistency. Gross I know. None of you will ever want soft-serve again. So, it's best not to exclusively rely on the sink. I favor getting your baby used to the potty and the toilet from very early on, so that it's no big deal when she's too big for the sink.