I've been using CM7 on my Samsung Captivate for roughly two months now and the change between it and the ROM Samsung/AT&T make available is astounding:
A) GPS actually works. On my stock eclair ROM it'd take upwards of a few minutes to get a lock if ever.
B) Stock eclair/froyo would give you 3G at best. The Captivate is capable of HSDPA which CM7 offers (but to be fair all you'd really need to do is flash a compatible radio firmware to enable this on stock ROMs).
C) Instead of randomly shutting itself off during the day and night, CM7 keeps on ticking 24/7 unless something catastrophic happens.
D) I'm not 100% what filesystems each use, but the stock filesystem causes a well known very noticeable lag that CM7 doesn't have.
E) Easily modified for Netflix usage.
F) None of those god awful AT&T apps forced upon stock ROM users. I hated those with my Palm Treo 680 and I still hate them.
To be fair most, if not all, custom roms are capable of the above. I have used others but typically found that my device would continue to randomly shut itself off, though this might be fixed by virtue of having gingerbread instead of eclair.