I presume you're talking about the Warm 'n Light series. I think they come in 600g, 800g and 1 kilo models. I have the Lafuma Extreme 800, a synthetic predecessor of the Warm 'n Light 800. I have just recently bought it, and only spent on night in it. For the $40 I paid, I think it's going to be a fine summer bag.
I suspect that the down variety are somewhat better than the synthetic version I have, though aside from the fill I'd expect similar construction.
I think that the Lafuma bags offer good value. Keep your expectations realistic. You aren't buying a $400 sleeping bag, so don't expect $400 bag performance.
If you buy, you MUST NOT store the bag long term in the factory (tiny) stuff sack. If you can buy the display model that's hanging on the rack, do so. Get a pillow case or large cotton stuff sack to keep the bag in when you're not using it.