The ritual candles to me are something that I thought were considerably good at first glance, but started to come around on them after further investigation.
The typical approach that I tend to take for any test that matters to me is to get to the +2 range in a 'standard' scenario. I mention 'standard' because my opinion of ritual candles changes when we start thinking about 'hard' difficulty and beyond. In 'standard' difficulty level play the +2 that I aim for on checks is going to succeed the majority of the time regardless of whether I draw a , , or (less so on the symbol). That means that the ritual candles are doing nothing for me in those situations that I really care about it. In the hard difficulty level you can still aim for a +2 and all of a sudden the candles are now bringing that -3 into the +2 range of success for the investigator and the chances for success go up significantly.
So as far as using it for standard scenario play, I am a little down on the card, but in hard mode play I am feeling positive on the card.
The other way to play when you get the ritual candle when there are several -2 symbols would be to then just provide a +1 to your skill check and possibly get an extra pull of the bag that would be in your favor. You still have a -2 or two in the bag that are now bad for you though. However, you do keep the negative effect of the symbol from affecting you at that point that the -2 didn't. By putting yourself at a +1 with the ritual candle you fail on -2 pull but might pass on a "-1 if you fail, drop a clue at your location" which is a better situation to be in.
If you have no hand-slots in use as is on some characters, the ritual candles can provide some improvements, but on the most part, I am somewhat down on it unless I am playing on harder difficulty levels and then at that point I find it quite beneficial.