Hi James,
The biggest difference between DMAA and caffeine is the amount of research done on caffeine. There have been many studies investigating the basic properties of caffeine as well and the use of caffeine in sports. Most studies I have seen suggest that caffeine is of significant benefit in endurance sports in particular. There are negative side effects from too much caffeine, from jitters through to several recent reports of deaths involving the large cans of energy drink.

For sport the AIS have produced a caffeine fact sheet which may be of interest.

One thing that does seem to change with caffeine is the required dose for performance enhancement. The AIS sheet above suggests 1-3 mg/kg bodyweight. For an 80 kg person that is 80-240 mg of caffeine. A Hi5 Gel+ has 30 mg of caffeine in, so you would be below the effect level with a single one.

The usual mantra re trial in training not racing stands here.

Also, be aware of possible side effects and take care!

edited to make link work