Windows Phone 7.5 Mango has multitasking capabilities, but what you get from these depends on the apps you run. Some apps – music players, for instance – get to run in the background, but mostly what you get is the same fast-task switching as before, with the addition of an app picker view so you can choose what you want to go back to.
If an app hasn't been updated to suit multitasking, it resumes from its frozen state just about as quickly as before. But apps that have been rewritten to suit the functionality resume more quickly, and they can also get Windows Phone 7.5 to run background agents for them to perform updates and the like.
It's similar to the way that Live Tiles work and a good compromise between making it easier to work with multiple apps and not undermining battery life. A handful of apps, including Evernote, have already been updated to run in the background, and you can go back to working with them as fast as if you never switched away.
Once you have a lot of apps that can run in the background, the new battery saver option comes in handy. When your battery gets low, this turns off Wi-Fi, push email and background apps. We found we could set it to turn on automatically and squeeze extra life out of the phone without ever really noticing that any service was getting turned off.