The problem with these things is that arguments on both sides are skewed mostly by marketing mumbo-jumbo and politized ramblings, case in point: the whole Hummer is better (for the environment) than Prius bullcrap that was based upon incorrect assumptions and old data.
Another one that always comes up is battery life, warranty and all, AFAIK
both Honda and Toyota provide comprehensive lifetime (10year) warranties, and have even begun to replace old batteries at no cost.
On the other hand I personally think the concept is flawed. The advantages of having an electrical motor on the side work best in start-stop traffic, exactly where the disadvantage of having +100kg of extra technology on board is most harmfull. The technology itself is very cool and works well (haven't personally tried it but my dad was impressed, he's a techhead and a petrolhead) In a GS450h the battery pack etc. reduces the boot to a rediculously small size. The start-stop advantages would be practical in taxi use, but the boot makes it unusable for that purpose.
In European conditions both the Prius and GS450h have been shown to be less efficient than Diesel powered European cars.
The GS450h was tested against both an Audi A6 4.2 (petrol V8 ) and a Mercedes E320cdi (6cyl biturbo Diesel) and lost to both in all aspects including consumption and refinement.
The Prius used more fuel and produced more CO_2 in direct comparison to a Citroën C4 HDi (4cyl turbo Diesel with particle filter)
The only reason they are being sold at all, is that they get tax brakes. IE A 10.000$ tax rebate in the Netherlands, or exemption from congestion tax in London.
Since you guys don't get Diesels (for now) and fuel pricing vs taxation is very different I won't comment on the economics, you'll have to do the math for yourself. Do know that for both ordinary petrol cars and hybrids, official fuel consuption figures are optimistic, not taking into account normal usage, airco etc. Some effects are more pronounced on smaller engines, others on larger engines. YMMV