Okay, to confirm, the chance of getting your shiny is increased depending on how big your chain is. :3
>You should register your PokeRadar and unregister other items. Using one more Key Item breaks the chain.
So this encourages the player to register the PokeRadar. Therefore, the PokeRadar creates no negative effects in chaining. :)
Remember, just because it is a 20 times better chance of finding a shiny, it is still 1 in 200 chance of getting a shiny. You know how on Serebii's Pokearth - it has the percentages on how often Pokemon will appear in that route? If it said 1% chance, that is literally 1 in 100. So double that - and that's how hard it is to find a shiny. Don't get your hopes up too high.
Also, once your chain passes 40 you can just soft reset from that point on since it could break your chain, and you'll have to do it all over again. xD
Hope I helped. :)
[Source][1] and Experience
[1]:
http://www.gameranx.com/updates/id/18136/article/pokemon-x-and-y-learn-how-to-chain-to-get-a-shiny/