the worry that the vaccine may sensitize dogs to venoms as allergens is idiosyncratic; i have become allergic to rattlesnake venom as have several other individuals i know who have worked with rattlesnakes. there is no doubt that colors my thoughts on the relative risks of exposing dogs to potential allergens. i don't know of any research on whether dogs are likely to develop allergies to venoms or to the vaccine.
the lack of research on the efficacy of the vaccine has been noted by a number other sources. i suggest you google for results yourself. i can point you towards statements from the vet schools at uc davis and csu that point out the lack of studies. i'm also linking a study done with horses on the equine vaccine made by the same company. it may require some knowledge of biology to fully understand, but i think it is fairly accessible. i am not aware of any other published research on the action of these vaccines.
to be more specific concerning my doubts regarding the efficacy of the vaccines, they are threefold.
1. rattlesnake venoms are a cocktail of numerous different proteins. the vaccine is said to be based on neutralized venom. I assume (but don't know, because I can't find that info anywhere) that this is venom in which the proteins have been fragmented by some means. have the fragments been purified? if so, what fragments of what proteins are used in the vaccine? basically the "cleaner" the vaccine, the less likely it is to develop broad spectrum antibodies to a variety of the proteins in venom. the "dirtier" the vaccine, the more likely it is to act as an allergen.
2. they say, i believe, that the vaccine provide some benefit for six months. i have a hard time believing that. when they inject horses/sheep to produce antivenins, they are injecting those animals far more regularly (weekly or so). the antibodies produced don't typically last that long in the body. i expect that after a month or two the number of antibodies an animal still has in the body is declining fairly rapidly.
3. just because the animal has produced antibodies does not mean that those antibodies are effective in ameliorating the effects of a bite. the number of antibodies available may be too low to be of benefit (i very much suspect this), the antibodies may be ineffective in neutralizing venom or the antibodies may only neutralize some of the proteins, but have no effect on others.
at any rate, take a look around and make up your own mind. i chose not to vaccinate my dog, even though i work him in areas with numerous snakes. but as i mentioned, i may be unduly sensitive to the possibility of allergies (and also, as a dog that has been around rattlesnakes for much of his life, he has already been exposed to venoms and may already be sensitized). if you are not as worried as myself about the possibility of allergy sensitization, there isn't much other downside that i can see. just don't depend on the vaccine alone!