I use a squadron of 3 buggies in combination with two deffkoptas in my army. They are very different in use, so keep in mind that they don't fulfill the same role as koptas. Buggies give you 3 twin-linked S8 shots at
AP 3, lot sof movement blocking (three buggies turned sideways can block off a lot of table), and they're more survaivable then deffkoptas, even being AV10 open-topped. The damage charts see to that

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I use my buggies as harasses and movement blockers, since even a land raider ramming it needs a 4+ on the damage table, or it's stuck against a 35-point model. If my ,opponent ignores them, they pop off 3 S8 shots a turn, killing transports, devastators,... If he doesn't ignore them,they take a lot of killing, meaning the rest of my army can approach unscathed. One last tactic is to use them as a bubble wrap for your battlewagons, preventing deepstriking/podding meltaguns from destroying it and being able to keep up at 12"/turn.
Hope this helped!