Impact is an utlity mod mainly used for its Baritone integration and decent-looking UI. The developers of Impact client are Brady, LeafHacker and leijurv. There is another version of Impact exckusive to donators.
Impact's combat modules are lacking. The CrystalAura is not good. Aura is decent and customizable. Not much to say about these modules.
Impact does have rather good movement modules, like 'Riding', which has EntityControl and EntitySpeed. Not much free clients have those features, which makes Impact stand out. Not much to say about these modules either.
Impact has decent visuals, although no SuperheroFX. The Nametags are rather customizable, and look nice. Chams also works well.
Conclusion: There is not much to say about Impact. The client is decent for players who dont take anarchy that seriously, but people who do should buy a client like Future or RusherHack. If you dont take the game that seriously nor do you pvp, Impact is a decent option.