@nevatik All very good advice given here. The XT can drive you crazy at first. Here's what I do that gives me very consistent results. Some of this has been stated already.
When planning online I use the HERE maps as it seems to give the most consistent results. Understand that when you import a route into the XT it WILL recalculate the route based on its settings (avoidances, faster vs shortest, etc.) I turn off the avoidances and select fastest vs shortest. I also use the GPX 1.1 route / track / poi format.
Put in enough shaping points so that the route is as unambiguous as possible. Be generous with them in areas where there are different ways to get from point a to point b.
As others have stated, when the route is imported the track is imported as well. Go to your tracks and make the track that matches the route visible, then when you pull up the route any deviations will be obvious. Play with the track's width and color to make it obvious when you're viewing it overlaid with the route.
Another very quick check to see if the XT's route matches your planned route in the planner is simply bring it up in the planner, look at the total distance, then bring it up on the XT and check the distance there as well. They should be VERY close. If not, check the route vs the track on the XT, make the necessary adjustments in the online planner, then delete the route and track from the XT and import them again. After doing this a few times you'll figure out what to look for (where to put your shaping points) and it will become a lot easier. I just planned a 3 week trip that I'm starting on tomorrow with my brother that includes 19 days of riding, and I only had 1 route that I had to modify and that was because of something really dumb that I had done leaving out some obviously needed shaping points.
The other very important thing (to me) that I'll mention is that once you have the route roughly laid out with the shaping points and via points, bring up the waypoint list in the online planner, select the first waypoint, hit the zoom button, ensure that it's exactly where you want it, move it if necessary, then click the right arrow to advance to the next shaping or via point, adjust as necessary, and continue through the entire route looking for any adjustments that are necessary. Move the shaping or via points away from any intersections that may cause confusion.
This can be annoying and frustrating at first, but with a little practice you'll figure out what the XT is doing and you'll be able to force it to do exactly what you want it to do!
Steve