I have Scanner Pro. I haven't used any others so I can't really compare. However, I've used Scanner Pro both on my iPod Touch and on my new iPad. I do a lot of genealogy research and scan old documents, handwritten ones as well as typed documents and photos, both B&W and color. I've even used it to scan images on microfilm readers.
It does everything I need it to do. The color photo scans aren't perfect, but I have been able to make acceptable prints from the scans. With the iPod Touch, it's much easier to hold it steady while taking a photo. The heavier iPad is not so easy to keep steady but the image stabilization seems to do a good job.
The higher quality camera in the new iPad makes for much better quality scans, so I'll finally be able to get rid of my CanoScan scanner and the 7 year old PowerBook G4. I've kept both until now because Canon didn't offer drivers for Intel Macs.