The temple guard by themselves are not bad, they are str 5 and are decent but a bit expensive without a Slann. The Slann makes them Immune to Psychology and Stubborn which means you'll probably be there until the last lizard.
As for Finecast, it is a Resin/Plastic mix... very light weight. I caution that so far I have seen very few Finecasts that I actually thought were better than their Metal counter-parts, and this coming from a person very against Metal.
This discussion might be better elsewhere but here's my quick thoughts:
Pros - it is light weight, requiring less pinning than a metal model. Also the light weight means there are less tipping problems than previously top-heavy models (oh, hello Slann). It does not shatter upon hitting the ground (metal models with lots of parts seem to have shattering problems). I have a feeling that there will be fewer problems with flying stands and these models. Final note: they are really nice looking, some of them have more quality and better detail than their metal counter-parts.
Cons - Some of the mold lines are really bad on these, and in a few rare cases the mold must have slipped so the two halves don't match up perfectly. Where there were gaps before, there are gaps still. Some worse, some not as bad. Lots of flash, but it usually breaks off by bending it with your finger (rather than needed clippers and hobby knife). Sounds like that should be a good thing? Unfortunately this also means some bits will also snap off that would not have snapped off metal models. More expensive.
There's good and there is bad to the Finecast. It is a matter of preference. My suggestion: get a model that you've been meaning to buy for a while and see if you like it. If you don't, no big loss because at least you got something you've wanted!
EDIT: forgot to mention, everyone I know who has gotten Finecast (at a GW) have been able to exchange their product if there were any problems with it. In fact, at the store I frequent, the staff prefer if you open it at the store and check to make sure everything is cool before you leave because they will exchange it on the spot.