The Jem'Hadar IMO were a pretty compelling entrant into the ST franchise, and I like how they designed them, gave individual character personalities, the special relationship with the Vorta, etc.
Of course, I always wondered what could have been done if ST-constraints were removed, for instance having them not be humanoid (although the rhino comparison was a pretty good one). If the Founders truly wanted to create the ultimate, most fearsome and effective fighting force, I think they could have bioengineered much more than what they ended up with...or maybe they did have more of an attrition/save resources model which resulted in a more humanoid-style, controllable footsoldier.
However, imagine if for instance the Xenomorphs from the Aliens series were used ("the ultinate biological weapon")...and there were no easy weaknesses (for some reason later on Dominion clones, Vorta and Jem'Hadar, could sometimes be killed with a twist of the neck). I would have also liked to see the full Dominion potential, more than just 3 species, showcased as a full "anti-Federation."
Anyhow, they were pretty effective, it's just that the line "and then they send in the Jem'Hadar" from Vorta Eris implied all sorts of menacing creatures. Especially when compared to those throughout the scifi universe.
In another ST-constraint, I'm guessing that it ended up being that Klingon warriors are about equal or close to Jem'Hadar soldiers, given the added gravitas and weight the former have in the ST mythos.