Most vendors do a good job of win/loss, especially when compared only to what your team is able to do in-house. Garter's research about how flawed win/loss answers are when they come from inside the company is 100% right. However, what I've learned from standing up win/loss at a few different companies is that you want to make sure you trust the elicitation skills of the person actually conducting the interviews. Folks who gather other types of market intelligence and are used to gleaning insights from willing and sometimes unwitting primary sources will often bring home the most insightful data because they are well versed at how to make the most intrusive question seem benign. If they can't get you what you can't get buyers to honestly divulge yourself, then why engage an outside vendor?
The first time you do win/loss you should (hopefully) have all sorts of new insights that you can act upon as a PMM and PM team. The next time you do a round you are going to hear 80% of the same information, and maybe 20% will have changed. Don't be discouraged! That 20% in many cases is more important than all the first round information combined because it's telling you where the ball is going, not where it's been. That's the strategic gold that gets you invited to present consistently to the executive team as a strategic partner and ultimately consulted for strategic decision support on M&A and other strategic decisions of huge consequence to your company.
Good luck!