Question about SenseUI.....

Coming from winmo, i was fed up of coverups upon coverups hiding a OS with new versions of standard features and still having the native ones hidden in the background just to make it enjoyable....

Now.... since Android is completely open source, Is HTC's SenseUI a coverup only having the standard Android UI under the hood, or did HTC REWRITE the standard Android apps and replaced them with theirs????

From this i mean, is HTC's version of the Album, Music, Contacts, Browser etc. an add-on having the vanila versions hidden somewhere like they do with winmo and can be reverted to, or are they ACTUALLY the classic apps redone by HTC???

I really hope HTC rewrote the OS cuz it really works well together (somewhat of a HTC OS all together)