I have used imagemagick/GD previusly, but there are some hassle involved in setting up this for each project/client (cron job for cleaning up thumbs, space considerations, not all ISP I use for clients have GD installed etc)
I agree that writing an ad-hoc image scaler is simle.
However, writing a quality general one that handles different formats, image quality, aspect ratio, smart cropping etc..is not trivial.