Like Deputy above, I run the Vortex - directly mounted to my scope tubes, on dang near every rifle that I own. I place them so that they are visible to my Left (non-dominant) eye.
I am aware of the rail sections that have these imbedded into the rear-most portion of rail, as well as the Spuhr mounts, many of which have the bubble level imbedded into their bases, just above the picatinny interface.
Let me be absolutely clear in that I do not think any of these are poorly made or bad options for other shooters, but for me, any level that is placed directly under the ocular lens of a scope seems to be in an odd spot for ME while actually on the gun. I know that many folks really like these models and have great luck in shooting them well.
For me, it just feels much more natural to be "on scope" and finding my hold, then change my focus to my non-dom eye and verify that I am level, then transition back to my dominant eye and complete the shot. The scope tube mounted bubbles seem to be in the perfect spot and cause almost no change in my line of sight when I am transitioning between eyes.
The other major advantage to the scope tube mounts for me, is that if you really take your time to level the reticle and tube, the level can stay on the tube if you want/need to swap scopes around to different rifles. I end up doing this a fair amount, and I end up with a plug-and-play scope/mount/level package that easily transitions between rifles.
You may end up preferring the rail mounted versions.