I'm a big fan of looking into a purchase as much as possible, but how do you know who to trust or what to pay attention to? Here are some tips for wading through the murky world of online lens reviews:
1. Screen shots. You can't really trust jpgs posted online for review. There are just way too many factors that go into compressing and uploading to sites and the final quality. It is far more important to pay attention to what the reviewer says about the product that the actual images. Any lens can produce an image that looks good at 640x480 resolution.
2. Is the reviewer a photographer, or a reviewer (or both)? If they are a reviewer getting paid to do reviews or supplied products to review their impression of a new piece of equipment is limited to a short amount of time with it. It is important to ask yourself if they took photos with it that will be similar to your subjects.
3. Test charts don't tell the whole story. Sharpness is just one of many aspects of a lens' performance. CA, fringing, vignetting (dark corners), distortion and bokeh are just a few of the optical properties of a lens. Quality of manufacture, AF performance, maximum aperture, zoom creep, minimum focus distance, weight, and other physical factors add to the list. My personal experience with lenses is that the black and white test charts may be the worst place to start when evaluating a lens. Some would argue that only prints tell the final story, but if you submit to stock sites then how it performs on screen at 100% is important.
4. Sample variation. If you get into reading reviews you will ultimately hear about how a lens has varying quality, that you may need to test 3 or 4 copies of a lens at the store to find a good one, etc. There is clearly some variation between lenses, and that is too be expected as a manufacturer can't have perfect calibration. There are small variances in the alignment of the camera body (mirror, AF unit, sensor) as well as the lenses calibration. Hence why modern bodies allow the user to adjust things. But that isn't the whole story. You have to learn how to use a wide aperture lens, for example. You may be used to locking focus with the centre AF point and recomposing - that may be fine at f/5.6, but with an f/1.2 lens wide open you may move a little while recomposing and throw things way off. Another catch is people who shoot the ruler on an angle to determine front or back focus - the AF point isn't exact either. I did a recent test with my 40D where I focused on a spot on a plain wall and it was still hitting focus on the spot with it completely outside the AF point in the viewfinder...
5. Expectations. When recently looking for a wider (28-35mm prime) I found that just about every lens reviewed had the comment "we expect more from a prime lens". It just turns out to be hard to make a wider angle lens that is optically good from f/1.4 to f/16!
My personal take on this topic is that you should read the optical reviews and see what they say - if you can't find anything positive that is probably a good indication. Then take a look in Flickr groups for the lens, and see what kinds of shots people are taking with the lens in question. That says a lot to me... Certain lenses just have galleries full of shots you wished you'd taken, and others are a bunch of ho hum shots with people gushing over them - those are the lenses I try to avoid :) Bottom line - don't take it too seriously as there is no perfect lens. Here are some sites to try: