Unless your dose ratio is way out of wack, I consider this to be a very good combination. In fact, if I'm taking anything other than cocaine or a threshold-to-light dose of dextroamphetamine, I would rather have a benzo to go with it. A low dose, 10-20mg diazepam or equivalent does the trick just fine - I feel it barely reduces any euphoria or central stimulation, but it does wonders for any possible anxiety, paranoia, and particularly muscle tension and peripheral stimulation.
I personally need quite a lot of sedative-hypnotic benzos to effectively counteract an active dose of most stimulants - they are better at getting to sleep 2-4 hours earlier than normal during the comedown, but while I consider the stimulant to still be in its active and not its crash phase, I would usually require benzo doses which straddle the amnesic frontier for them to seriously affect stimulation (and compared to most others around me I consider my stimulant doses quite conservative, when for most other drugs the converse is true). In the rare case that they do make me too dulled or sleepy, I take a tiny bit more coke/speed/meth/whatever. In 98 cases out of 100, for me, benzos improve the stimulant experience, and remain your best chum when the stimulants leave the party.
At reasonable doses they aren't strong enough to ruin the experience. They "take the edge off" and I think most people would agree that on stimulants, that would sometimes be welcome for most of us. Stimulants + benzos are also fucking social steroids. I'm surprised my tongue doesn't turn silver.
