Having lived in Manchester twice for 6 months each time, I used taxi's in Manchester a few times and I have to say I am impressed.
Taxi's run the same as London ones, with big black cabs, these are great, much easier than a conventional car, with plenty of leg room and large enough for 5 passengers plus the driver, with the back seat for 3 passengers then two tip-up style seats behind the drivers cab. You have plenty of space for large suitcases, take this from someone who doesn't travel light, you have ample space in the back for passengers & large luggage. The cabs also rather comfortable too, they are also fitted with security locks on the door, which automatically turn on when the cab is moving.
I must say drivers in Manchester are very friendly, obviously I've not drove round with all of them but the ones I have come across, are not arrogant or rude, they engage you with light conversation, which I admit can be a bit tiring if you can't be bothered to talk, but they are just being friendly. Makes a massive difference from where I am from, some of them taxi drivers are scary, so Manchester is a refreshing change from home when it comes to using taxis.
They are fairly priced too, unlike in London, I could travel quite far for about a tenner, they are easy to pick up too, either call one of the many operators or if your in the City you have taxi-ranks all over where you can pick one up pretty much all day and night, only one time I had a problem was on my birthday night out with work, at 3am ended up waiting about 1 and a half hours in temperitures of about -2c for a taxi.
Would still deffinately recommend a taxi if your in Manchester.