I'll assume that you're asking specifically about sidechain compression of a bass or pads using a kick drum as the input. The purpose of this - which is but one specific usage of sidechaining - is to 'duck' bass/pad parts of similar frequency content to the kick. This keeps the mix clear and stops things getting muddy. It can also be used less subtly to get the characteristic 'pumping' sound you hear in so much house music.
Is it a big deal? The last sentence of the previous para should be a clue, but yes. If you're making contemporary house then you really want to understand how sidechaining works. Try searching the forum - there are approximately 5,000,000 threads about this already.
Is it OK, to use a drum loop, with hi hats, claps and kicks... and then sidechain the whole thing? Im pretty sure im talking about sidechain compression here :p im kinda learning it now :s but is it OK? or is it just BAD?
If you're talking about using the drum loop as the sidechain source and triggering compression on the other elements with it, then no, wouldn't recommend it. It's easy to build a "ghost kick" (or it can be a hi hat or something.) that is muted but is what is triggering the sidechain on other elements. That way you can have flexibility and can sidechain compress something without the trigger sound actually being heard.
This is from an Electro/progressive house standpoint.
Sidechaining is Super important. It will allow the kick to come forward and not be drowned out by overbearing bass frequencies. Both drums and low end bass synths share many frequencies so when they are played together it makes your mix sound muddy at times. (This should also be fixed with EQing too)
I create a track that is made of only kicks on time with my song and is muted. It goes through the entire song and everything that is set to side chain is triggered from this sound. It is bad if you set the side chain to trigger from your whole drum section. It will make everything constantly trigger when you really don't want it to.
It takes a lot of practice to learn how to sidechain properly. Songs I've made 4 years ago sound horrible to me because I found I had too much attack and to little decay on my side chaining.
Take the time to learn how to properly sidechain and try out different parameters in your DAW.
I've heard some amazing songs with great potential sound horrible because there was too little or no sidechaining.
I even think in dubstep its important to have at least a little sidechaining to make the kicks and snares come through