the app i think is most worth investing in for is
Bitesms
It outshines the standard sms app... you can quick compose... quick reply etc check it out...
Another good app is faceclockup.... it speeds up the iphone big time
Also multifl0w is a really good multitask app just like on the htc phones..
Sbsettings is good for quick settings.. i.e you just scroll your finger at the top you can switch off wifi... 3g or turn it on etc, rathen then going into settings etc...
Folder enhancer is a pretty app... it expands folders and you can have more then 12 apps inside.
Safari download manager is nice.. you can download zip files from internet (on the phone) etc....
you will need ifile alone there too... which you can browse all the folders on the iphone ( just like windows explorer basically)
Safari tab is again a nice way of scrolling through different web pages..
Barrel is nice... you can animate the way you scroll through pages..
Graviboard is a nice app for you to show off to your friends...
Winterboard is a must to install themes
CYDIA is your best friend.. go through each app when you have time.. it will give you a description before you download it... definatly worth checking out