For: Light!, Cheap, Sharp at F1.8, Great colour and contrast, Very useful for low light indoor portraiture
Against: Doesn't focus anywhere near as fast as L glass lenses, No lense hood supplied with it
Canon 85mm F1.8 USM
After using the 17-85mm kit lense it made me realise just how much of a zoom 85mm is indoors !! But when you've adjusted to the fixed focal length of 85mm you find it to be pretty much perfect for headshots in indoor and outdoor social situations. You dont have to stand too far back or too close to your subjects ( which is nice for people who dont want the lense directly in their faces ! )
F1.8 really is a fun thing to master. It will give you a much higher shutter speed which is always useful and also in the right situations it will produce a gorgeous bokeh too equally as good as the 135mm F2. For anyone not familiar with F1.8 you'll need to be careful with your closeups as you'll have a relatively thin DOF to work with. Many of my earlier shots had 1 eye in and 1 eye out of focus. The solution is to take a step back, crop the image or increase your F number.
But once mastered, F1.8 makes for fantastic portraits. Especially at night! F1.8 will turn any city lights in the background into gorgeous circles of light.
The focus system is good, but slower once you're used to L glass. It hunts a little more than the L glass lenses but at this price it's still reasonable.
This lense is sharp at F1.8, even sharper at F2.2 with a good contrast and produces great images considering it's so much cheaper than other lenses in the same league. It's almost as sharp as the 135mm F2 and that's saying something for a lens which is 60% cheaper.
Another huge plus is the weight - it's light. VERY light. So you can not only hold this lens for long periods of time but you can use it on gorillapods ( lightweight tripods ! ) and also it makes it realstic to take handheld shots at 1/125th which for a non-IS lens is again very useful.
So to wrap things up, it's a cheap, light but very good portait lens with a focal length you can use inside and outside handheld. If you're starting out taking photo's of people they are always floating around ebay 2nd hand for very reasonable prices (as the first lens when you've spent all your money on a new DSLR always stings a bit!) and when you're running out of light, F1.8 will get you many shots that F2.8 cannot.
As with all lenses, if you can then try before you buy !
Canon 85mm F1.8 USM, ISO 320, F1.8, 1/5000th
Canon EOS-1D Mark III @ 85mm, F1.8, ISO: 50, 1/2500th