The 830 is great, the 840 is faster in benchmarks (though it's unlikely you'd notice the difference) and had some issues with pre-release firmware which they fixed, I'm still avoiding OCZ for now due to the extremely high RMA rates on their older drives and I've nothing to say about Crucial.
I have myself 2 Vertex 4's and both are great. I put one in my friends computer and its done great so far as well put a M4 in my wife's computer and its doing good as well it's about 1-2 secs slower the the Vertex 4 but its done its job so far.
m4 and 830 are about on par as far as performance goes and are certainly the two front runners in the SSD market today. 840 is a downgrade from the 830 (as far as I know)... the 840 Pro, however, is a step up but probably not worth the price that it demands.
While I do agree with the consensus (830 or m4), I've been using the OCZ Vertex 3 since November of 2011 and I've not had any problems. Also recently picked up the Vertex 4 and it's been great thus far.
The new OCZ Vector might prove interesting as well. It offers great performance but it's to soon to say if there will be any problems with it. The 5 year warranty is quite nice as well.