Live CD's/DVD's are alright, but only if the band played really well. The more you listen to a live performence, the more you pick up on the mistakes and out of tune singing. The Lamentaions DVD sounds worse everytime I watch it.
But that's just me.
Having said that, i'd still pick up an official Live Opeth CD if they released one, as long as it wasn't the same Lamentaions concert again..