JuiceDefender can typically help battery life if you are a light to moderate user. If you're a heavy user, or like to turn you phone on frequently (every 10-20 minutes to check things), it's most likely not going to dramatically improve your battery life. In fact, it can make your battery life worse if you're a heavy user or constant fiddler. I've been using it (including JD Ultimate) for several months now, and on days when my use falls into the light-moderate range, it approximately doubles my battery life.
As far as your email goes, is it POP email or IMAP? Did you set your email client up in the Apps section of JD? If your trying to set it up in the Apps, I haven't use that, so I can't help you there. I think in most cases that's used for apps like Pandora, Google Music, etc. All of my email addresses are IMAP, so I just have JD configured to turn data on every hour, performing a ping, and my email will come through. If you need your email in real time, then JD may not work for you. Or maybe someone else can help you in this area.
Here's how mine is configured, but I'm using JD Ultimate. And I've only listed settings that specifically have something selected.
Status
JuiceDefender Ultimate - enabled
Profile - Advanced
Controls
Mobile data - enabled
WiFi - enabled
Options - wifi preferred
3G toggle - disabled
AutoSync - ping
Schedules
Schedule - enabled
Frequency - 1h
Duration - 15s
Controls - default
Night - enabled
Start/End - 11pm -> 6am
Adaptive start/end - +2, -2
Peak - disabled
Weekend - disabled
Triggers
Battery - enabled
Battery threshold - 15%
Charger - ac
Screen - enabled
Controls - default
Traffic - normal, 15s
Apps - disabled
Location - enabled