Okay, so it costs an arm and a leg, or in my case a boat and an outboard, but fast glass makes a big difference to image making. My local camera shop let me try out a lens before making my mind up to buy. It only took a few shots to convince me. Back home on the PC and I saw images that, when converted to b&w had the look I used to get with my old Pentax ME and the 1.7 and 2.8 lenses I used to use with it. Deep joy!
'Pro' lenses come with other benefits too. They feel much more solid and smooth in the hand, rather like the way even 'consumer' lenses felt back in the seventies. The auto focus is fast and accurate. They are also sharp and contrasty. I didn't bother sharpening the photo below because the lettering was crisp enough straight out the camera when reduced. Click it to see it bigger and clearer as the blog messes photos up when shrinking them.
Used wide open, or nearly so, they also give that 'look' that sets top quality wildlife photos (for instance) apart from those that are good but 'not quite'. I think it's called 'good bokeh' these days, but out of focus backgrounds are so smooth, and well out of focus. Not wild wildlife, but the pic below demonstrates how this helps isolate the subject and enhance its apparent sharpness.
When it comes to lenses you really do get what you pay for. A pity you have to pay quite so much to get it.
PS - same story, different teller.