Those who are appearing for JEE 2016 should join a test series by now. There are a number of test series - FIITJEE, Resonance, Narayana, etc. Since I was enrolled in FIITJEE, I joined FIITJEE All India Test Series. FIITJEE test series is extremely hard - way above JEE level and at times it is demotivating. So, I would not recommend FIITJEE test series. I have seen a couple of Resonance test series papers and they seemed to be more relevant to me. Also, I have heard good reviews of Resonance test series from my peers as well. So, I would recommend you to join Resonance test series. For those who are aiming for a high rank may however choose to opt for FIITJEE because almost all top (50) rankers join FIITJEE AITS and so, appearing for FIITJEE AITS will help you evaluate your national standing better.
Quite a large number of you ask me - how much should I score in test series to get a top X rank in JEE?
Well, this question has no answer. Score depends on the toughness of paper.
Then what is a good score to check?
What used to be my score?
I consistently had 2 scores - 99.9 percentile and 99.8 percentile in FIITJEE AITS
What used to be my ranks?
My ranks in FIITJEE AITS varied from 2nd rank (once) to ~40. I even got 59th rank once after messing up a full test. Since I finally ended up with JEE AIR 33, I strongly feel that FIITJEE AITS gives you a good approximation of your real standing. Note that this applies only at the top ~100 rank level. After that, your rank in FIITJEE AITS will have significant error in approximating your actual JEE rank.
A strategy that I liked about FIITJEE was the way they took part tests. Before starting the actual test series, FIITJEE divided JEE syllabus into 3 parts. Then they held 3 part tests from each part. There was a gap of 2 weeks between 2 part tests. These part tests greatly helped me to completely revise the JEE syllabus in about a month. Moreover, I was able to learn a lot during these part tests. I realized that there are certain concepts at which I need a better command and so, I solved more problems from those concepts and discussed them with my teachers. I believe that it was this 1 month of preparation that really took me from a top 100 rank to a top 50 rank in JEE.
Regarding test series, I would like to give an important summary that should answer your questions.
I would personally recommend FIITJEE to all those who have been performing well so far and who are really aiming for a top 100 rank in JEE. FIITJEE test series is highly recommended for those who love challenging problems beyond JEE syllabus and those who are willing to learn new concepts at this juncture.
However, if you feel you have some backlogs, or you are not that good, or you feel you won't be able to get a top 100 rank, then please do not join FIITJEE test series. It will be highly demotivating for you. FIITJEE test series is really meant for those who have done proper 2 years of JEE preparation with zero backlogs and a good understanding of all the topics (that's how you define a top 100 ranker). Note that there are exceptions and please do not misinterpret my statements. All I mean to say is that FIITJEE test series is not meant for you if you have some *hole* in your preparation. I joined FIITJEE because I was very sure about my performance. I had been performing well consistently in class 11th and 12th in the FIITJEE phase tests and that's why my teacher asked me to join FIITJEE AITS. Also, it was a part of my FIITJEE fee payment and so, I did not bother to look at some other test series.
All the best