How practical it is to use an Android phone, bypassing Google completely?
Android comes with an Email app that lets you use any POP3, IMAP, or Exchange account.
Talk can be replaced by Meebo, eBuddy Messenger, and many others.
Voice is a fairly unique service but some features can be sort of replaced -- you can use a SIP phone number and an app like Sipdroid for calls, or something like Kik Messenger for text-like instant messages (think BlackBerry Messenger).
Searching "Maps" in the Market shows several results that look promising, same with Calendar.

The Email app is very, very buggy and low on features.  I'm confident there are other mail apps that are better though.  On the other hand, Talk is also pretty buggy; I find eBuddy to be much better.  And many people have problems with Calendar.

When it really comes down to it, a quality app is a quality app, whether Google wrote it or not.  Android doesn't seem to be as biased toward Google's apps as Windows was towards Microsoft's apps.  My hodgepodge of Google and non-Google apps works well for me and I'm sure you can get a set of apps that will work at least as well overall as Google's apps do.