Yeh I just completed JLPT n5 in December, and until I got my test results I decided to cram for 9-kyuu which was held this past Jan.
It definitely paid off, my vocabulary increasd 2-3x fold. I took he test, it was with a bunch of kids and I was the only foreigner but it was fun. I'll take 8-kyuu in June. I also find it a lot easier tacking the core 6000 on smart.fm since they have the 6000 broken down into kanji kentei steps of 200 each. I think for 8-kyuu they've got like ~750 vocab words in the core 6000 which use the 9-kyuu kanji.
On another note, I got my results back for JLPT N5 and I passed. My plan is that while I'm studying for the JLPT N4 I can study for 8-kyuu. Also by getting a head start on kanji,I can say I'm more comfortable with kanji and vocabulary, giving me more time to focus on my weaknesses, LISTENING and READING. If things go well and I still on track, then I'd like to complete and pass 7-kyuu and JLPT N3 by the end of this year. An impossible goal, but one worth striving for is to be able to pass JLPT N2 by next July, and 6-kyuu by next Jan & 5-kyuu by next June. Of course if I fail a level on the Kentei (which realistically I will, I'll still be learning). Plus since I'm stuck in Japan for 4 years thanks to the US Gov't I may as well learn the language while i'm here.
Also it appears that when I inform Japanese people in Japan that I've taken the nihongo-nouryoku-shiken (JLPT) i get strange looks, but when I told them I was taking the Kanji Kentei (kanken) i got remarks like "sugoi!", "Ganbatte!", "jouzu!"... So like a dog, I tend to respond better to positive comments like that. Now if they would only give me treats in the fashion of sushi and sake...
If anyone out there is considering taking it I would recommend it. Also I've got some books to recommend that I used on my blog in addition to the Nintendo DS game I used.
http://andrew-jlpt.blogspot.com/