Feature Set:  

Acclaim Images offers stock images and some stock illustrations. They offer a variety of resolutions and some media management tools such as a lightbox. They allow you to email the image before purchase and you can copy the watermarked image for experimentation.

Some of their isolated images come with a clipping path, however, they cost almost $80/ea whereas for less than $100/month PhotoObjects.net offers pre-masked images.

Search/Sort Options:  

Acclaim's search function offers several filters including by license and image type and it can search by keyword, photographer or by topic.

Quality:  

There is a wide variety of quality levels available through this site, ranging from nearly amateur to experienced photographers.

Selection:  

This service offers many photos, as well as the option to purchase posters or prints. However, they do not offer a wide variety of concept images or popular collections. They also do not offer special interest images such as editorial or photojournalism.

This is a good service to investigate if you are looking for a moderately priced one-of-a-kind image. However, this is not a good site for those looking for isolated or photo objects or concept or digitally altered images.