As long as you dont overdo it, jailbreaking is a MUST in my opinion. i have infinifolders, so I no longer need 2 different game folders, 2 different business app folders, etc.., I have bite sms, which is 10 times better than the stock sms app, I have "poof" so I can hide the stock icons I dont like, My3g so I can use facetime anywhere and watch anime on crunchyroll app whereever i want, without wifi restrictions, and i have mywi so I was able to use internet on my laptop while on vacation in orlando for free (mind you I have unlimited data). Oh yeah, and I have longitude set up to track my phone every two hours, it doesnt even have to be running in multitasking, it just does it and I can go into google maps and see where my phone is at all times.
With just those things installed, no sb settings or anything else, my battery life is the same and the speed is the same. I would never go back to stock. The ONLY difference I should warn you about is infinifolders, when you have a folder with a lot of apps, there is a one second delay sometimes when opening a folder, but that is it. To me its worth it to keep my apps organized nicely and not have to worry about the 12 app limit.
Edit: Forgot I also have User Agent Faker and am streaming Netflix movies on my phone, LOVELY 🙂