The short answer to your question is "no, they're not very good."
However they can be used strategically if you're playing on a big enough and open enough board to tie up or threaten flanks and rears. Very useful especially in larger games. Throw them out to lurk, harrass and give the enemy that -1 command penalty. Try to ride behind enemy formations to block off retreats in anticipation of the Orc Infantry/Boar Boyz charge.
They are useful in penny packets in a combined arms approach, to lure out enemy heavy units, to threaten war machines or to ride out on suicide "hey everybody fire at me, I'm the closest unit" missions in order to screen your main body advance.
Use them like a napoleonic light cavalry screen and they will pay for themselves. Use them in a shock role and watch them melt away!!!
My two penneth.
Dave