Hey,
The Chaos Repulsive Grand cruiser is pretty good, a tad slow for the Chaos fleet but a good addition to the IN arsenal (eg with the right fleet list etc).
The IN Vengeance is good.
The IN Excorcist is good.
2 Vengeances is a good concept, add an Emperor BB and you have an awesome 'abeam' line.
Vaaish uses this setup.
They work well in pairs, yes.
The Avenger is great in concept.
the BFG:R variants are... ehm .. don't remember, better priced I guess. haha. But the FAQ2010 Vengeance/Excorcist are good already.