This phone is very attractive. Its features are also attractive. But some minus points are also there in this phone. And you can never compare this phone with nokia 2100. The price is perfect.
I use this phone even in my classroom. Its too small that it justs fits in my hand and you can even talk while the lecturer is seriously taking some chapters.(I did it several times, was caught on two occasions).
Battery is never a problem in this phone.
If you wish to consume more then if you turn off certain features, your battery will last long(background picture, light etc).
But it gets heated up too soon.
I will personally recommend you to take a nokia 2100 instead of going for sony ericsson.
In nokia all models you can change the appearance easily, just by changing the panel cover you can make your old phone new in no matter of time. But in sony ericsson T100 you can change the cover excluding battery (battery is a different part in T100). So if you have bought an icy blue colour and if you change your panel, your phone will look ugly with your battery colour being something different from your panel.