I was a WM5/6 user for quite a few years (at least 4+ years). I heavily hacked my WM phones to get the functionality I craved.
After fooling around with various Android ports to my HTC Titan, I decided to give the TBD Verizon Android phone (aka Droid) a chance when it came out. Since that fateful day, I haven't had a single regret. The Droid (and Android) is absolutely amazing.
Prior to my Droid, I had a couple of almost totally unused GMail accounts. I was resigned to the idea that to get the full functionality out of my Droid, I needed to move into the Google "cloud".
I have been amazed at how well everything flows together, now that everything syncs/baselines off of my main GMail account.
I'm not sure how functional the phone will be if you don't have a GMail account. Here is my (probably inaccurate) guess of functionality degradation you will face by not using a GMail account:
1) The Google Market may not allow you to purchase any apps. App purchases are paid through Google Checkout - Which is associated with your GMail account.
2) EMail integration is obviously very well implemented with Google. When a message is sent to my GMail account, it seems like I get it just a few seconds later on my phone. You can also set up your GMail account to poll any of your already existing email accounts from other providers. While not updated as fast as your GMail account, it still operates very well, and also color-codes which email account your messages came from. If you don't want to use GMail, the K-9 Mail app can work as an alternative to the GMail app.
3) Syncing with Google Contacts and multiple Google Calendars won't be available to you. The instant syncing of contacts/calendars is *very* useful, and works totally seamlessly. Again, updating a contact (or event) on the web (or your phone) nearly instantaneously updates the phone / web, respectively. You don't have to worry about losing contacts or calendars anymore. If you already had a GMail account with Contacts & Calendars, all that information is available on your phone within a minute or so after you have powered up your Droid for the first time. (Note: I do have some moderate gripes about how contacts are displayed & grouped on GMail & Android. I hope Google will be addressing these items soon). If your Droid gets damaged and you don't have a GMail account, you will probably have to manually reenter all your contacts / appointments.
4) Other non-Google "push" apps (such as Flight Tracker Pro) probably won't work since they need your GMail address to send their integrated information to your phone to update their app status.
5) Probably a bit of other stuff, but I'm too tired to think right now.
As far as my "who is watching my activity" paranoia level is concerned, I don't have any concerns at all about Google. I haven't seen a single instance in the months I have been using my Droid where Google appeared to be targeting any overtly obnoxious advertising in my direction. (However, if you are still concerned about ads, there is a hack to Android to block all ads). If I had any concerns, I would certainly trust Microsoft less (by a few orders of magnitude) than Google.
I'm not entirely sure how Google is making a lot of money from the Android adoption, other than having more devices natively using Google for searching (which can result in their advertising revenue). But from what I have seen, it is no worse than when you are using Google on your PC to do your web searches.
After using Android for months now, I can't ever see going back to WM6 again. WM6 was incredibly lame in comparison. Web browsing on Android is what I always hoped the WM6 platform would achieve (but never seemed to reach). Android is an amazing OS, certainly comparable to the Apple iPhone in almost all aspects, and exceeds WM6 in most aspects.
My advise would be to take the plunge and get an Android phone & GMail account set up. Try it out. If you don't like the OS (or Google cloud stuff), you can always return the phone within a couple of weeks for all the carriers, and go back to a WM6 phone.