I finally got my replacement battery (the one I got was faulty)..Here's my take on the Mugen Power Battery for the N900

Background Information on Phone usage:

• Phone is overclocked to 700MHz
• Phone set to 3G only
• Connected to internet 24/7 with Skype, Gtalk and Push Email via Nokia Messaging always on
• Multiple chats open, with several VOIP calls made
• I also tweet using the tweego app

(basically I'm kinda of a heavy user, and coming from a Blackberry, I tend to IM alot. Also I do browsing with multiple browser open at a time. NOTE: I do not use the phone as a music player)

The phone would be disconnected from the charger 5minutes before leaving the house (around 9am). With the stock Nokia battery, I would have to set the phone to use GSM data and only switching to 3G when browsing the net, and the phone would show the low battery around 4 or 5pm.

Currently with the Mugen Battery, disconnecting the phone from the charger 5minutes before leaving the house, I could set the phone to 3G connection and the low battery warning would popup around 8pm. Heck I could even play Angry Birds with sounds on and not worry about the battery

Overall, I must say that I am happy with the Battery. It definitely give you more time to use the phone for other stuff like IMs, browsing the web, VoIP, etc.

The size of the battery with the case makes it quite nice to type with the qwerty keyboard. If you have the otterbox case, its not much of a difference in size between the two.

I could do more testing if you guys need me to on top of the crazy amount of IMs I do on this phone and let you know how long the battery last me for

***
Edit: I added some more things I do with the phone on a daily basis. man my N900 is taking a beating isn't it? Also added the fact that the phone is overclocked (almost forgot that)