DCOLACE  Plaxo for Mac has never altered the data that is synced. What you might be remembering is that older versions of Plaxo for Mac would create custom labels for things like email and phone numbers with things like "work 2" and "home 3" to match the multiple phone and email slots shown in the Plaxo Online interface. The past several versions of Plaxo for Mac have done away with these numbered, custom labels so that software that works with the standard, Apple provided, "Home" and "Work" labels will work correctly.  
(Version 3.12 v839)

DCOLACE  We are completely committed to developing Plaxo for Mac to be the best solution available to Mac users to keep their data in sync across the web, and more. I use and depend upon it myself on my three home Macs.

Our Plaxo for Windows software has a couple of years head start as far as features and performance/reliability work. We started from scratch with the Mac software so that we could provide a custom solution that would integrate seamlessly with the Mac.

I'm sorry for any peculiarities you've run into in using Plaxo for Mac over the past months/years, but I think every release has marked steady progress in features and reliability. I hope that your experience with the updated release will bear this out.

Keep a look out for our coming release as it is one of the most significant releases we've done in quite a while in terms of new capabilities and reliability improvements.  
(Version 3.11 v636)

DCOLACE  You are certainly entitled to your point of view but I would argue that you are diminishing the usefulness of the Plaxo service by selectively employing it. One of the goals of Plaxo is to take the burden of contact management off of our members shoulders. With Plaxo running on your Mac, your calendars and contacts keep up to date without you having to think about it. From the web to your desktop (and likely to your phone) you shouldn't have to wonder if your latest edit (or Plaxo contact update) is where you want it to be.

Hopefully, we'll earn your trust so you can use our service to its full potential.  
(Version 3.10v616)

DCOLACE  Which version of OS X are you using? I've recently tested this feature on Leopard and I know it is working as designed.

You might be confusing the fact that we will auto-launch Plaxo when you launch the Address Book (all OS versions) or Apple Mail (10.4 and above) as well as the Plaxo preferences since it services the functions of those respective plugins.

Plaxo is intended to be a service to your system and really should be running all the time. When it isn't actively syncing, it uses virtually no CPU time or other computer resources (check Activity Monitor for a comparison of everything else running on your Mac).

If you don't like seeing the menu bar icon, you can simply turn that off while still allowing Plaxo to run.  
(Version 3.10v616)

DCOLACE  There is an option on the initial screen of the Plaxo system preferences to show/hide the Plaxo menu item.  
(Version 3.9v616)

DCOLACE  Actually, Plaxo has two way sync for calendar information so your iCal, Plaxo and Google calendar do stay in sync regardless of where the edit is made, but contact sync (from gmail) is currently one-way in to Plaxo.  
(Version 1.2b208)