I use one of them with my brushed speedo with no problems whatsoever. I know it works as I deliberately used the LiPo down to the cut off the first time I used it but in a race I've never got any where near it cutting in. In fact, in normal use I forget it is there! If you go back to using NiMh you can unplug it from the Rx and plug the speedo back inti the Rx which by-passes it completely.
When it does "stop" your car you get a bit of warning because it slows slightly as the voltage drops and then the drive goes. The steering works fine when it cuts in (so if you're mid corner you're not going to bin it!).
There are other manufacturers out there that do the same thing for a bit less so shop around Tel (I got mine when I was in the US as $2 to the £1).
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