From the BLW book this might help you decide:

False signs of readiness:
• Waking at night
• Weight gain slowing slightly
• Watching their parents eat
• Making lip-smacking noises
• Not going straight to sleep after milk feeds
• Small baby
• Big baby

True signs of readiness
• Sitting up with little or no support
• Reaching out to grab things and taking them to their mouth quickly and accurately
• Gnawing on toys and making chewing movements
• Starting to put food in their mouth ….if given the opportunity

Personally I am waiting till DS is doing all 4 of the 'True Signs' - mainly for me the idea of eating while not being able to sit up well doesn't sound fun to me - but also watching him with his toys I know that most food would end up in his eye rather than his mouth as he is not doing the "quickly or accurately" bit. (Oh and for me solids are more hassle than BF so I want to wait until I know he really is ready)