Nill,
If you are going to use the external drive for backing up your raw/tiff/jpeg photos, the compression ratio achieved is not going to be be worth the trouble since these file formats do not compress well using lossless algorithms. Aside from that, I have not encountered any stability or reliability issues with the NTFS compression before. I have used it quite a lot in the past when disk drives were much much more expensive per MB <remembers paying $500 for a 300 MB hard disk once upon a time, grin and sigh>.
BTW, many backup programs compress the backed up files (e.g. Norton Ghost) but for regular copying of files, this would not apply obviously.
Just my $ 0.02...
Cheers,
Cem