if u want speed and dont care for other thing like video call, video recording , camera , battery drain(so far is ok can last for 24 hour with normal usage) go for cm7.
darky  ..... stock rom is much more stable. 

My take.. CM7 is not good. It's great!! MISSING some small things like office app, video calling, smooth 720p video recording, low battery drain, fully working FM radio and TV Out etc. If you need these, best not to consider CM7.

CM7 is really suitable for those who want to take their SGS to the next level of Android sweetness, so to speak, but you must have time and motivation to read lots of tutorials, FAQs, and forum postings before u think about giving CM7 a try.

Note : I follow the CM7 thread @ xda-dev religiously, and noobs posting dumb questions there get royally fcuked on a daily basis. I see a lot of stupid questions here i.e. people who either have reading/comprehension problems or have not even read the info compiled in the first few posting of this thread, I can't even imagine what would happen if these were posted @ xda-dev. Credit must be given to the very tolerant and patient sifus in LYN.. 

u can get it here --> WiFi Cast Lite