I personally love Pokecheck's ability for me to transfer 4th Gen to 5th gen via PokeTransfer method, with just 1 DS. It is very convenient for me. It is also extremely useful for getting exact IVs and EVs and not ranges from any normal IV Calculator. Again, it is also really useful for finding my SID for RNGing, and also checking Pokemon for legality.
On the other hand though, Pokecheck has extremely hindered trading now. Back in the day, you met up with someone and set up a meeting to trade what Pokemon you desire, or you could use random GTS and enjoy it when you get a trade out of it. Now with Pokecheck, literally anyone with a WiFi connection can get flawless IV/EV/Shiny Pokemon with just a few clicks. In my opinion, it's cheaty and not right for a person who actually takes their time to RNG/hunt.
That is just my own opinion; I'm sure many people have other views. But what is true is that Pokecheck has extremely changed how trading works, and I'm afraid it will never go back to the way it was meant to be.