The killer feature for me in Windows Phone is Tellme over Bluetooth. You can have an entire text message conversation without ever picking up your phone or looking at it. This is great while driving, if you have a BT stereo or headset. It's one of the reasons I take my Lumia when I travel.
Nothing else even comes close. iPhone requires you to hold the home button to initialize Siri to read your messages and again to respond. The closest app on Android that duplicates this is Vlingo and, while it will automatically read the incoming messages, you still have to unlock the phone and tap the screen to respond.
Tellme over bluetooth is something Microsoft should make a much bigger deal about, in my opinion. Voice-to-text is definitely inferior to the other two platforms, but the automatic prompting that allows you to have an entire text message conversation using your voice is unbeatable.
I agree that while in theory the service sounds great, in practice it's not that amazing. I have a BT stereo in my car and play music through the phone all the time. I'm very well versed in "using" the service you're describing.
1. Listening to a song, text message comes in, tell me says "Would you like me to read it or ignore"
2. Say "read it". Tell me reads it.
3. Tell me says "would you like to respond or are you done"
4. Say "respond". Tell me beeps to go ahead talking.
Here is where it gets sketch. The voice recognition on tellme is just not that good. Anything beyond "ok" "Be there soon" "Whats up" etc. has about a 80-90% chance of being wrong. I'm sorry but that is just the truth. I've used this in the car on a Samsung focus s (wp7) which i had for a year and several nokia 920's (kept getting it replaced) and on every single phone I used tellme was equally bad at reproducing what I was saying.
I've disabled that feature as it's annoying to keep interrupting with my music with text messages that I know I can't even respond to correctly by speech.
I am not really sold on voice as a useful tech. This car situation is probably the only place I would ever talk to my phone. People who use Siri look and sound dumb, and usually have to repeat their question 5 times. Google is a little bit better from what I've seen but most of the time opening your browser and doing a search for "what day is valentines day this year" will be a quicker process than repeating the question to your phone 3 times before it "gets it".
Until voice recognition is literally perfected I will probably never use it.