I finally tried Speed's hot dog truck today, figuring the nice weather would guarantee that the cart would be there. It was there!
Overall it was....just decent. Not amazing, not totally disappointing, just fine. Sorry! I'm glad I finally had the GREATEST HOT DOG IN THE US OF A (tinyurl.com/3n8llk), but for me it is not worth a repeat visit. My three complaints: (1) way too sweet for my tastes; (2) too expensive for what it is (I know it's GIANT, but $7 is too much for me to spend on a hot dog--I'd rather drink my lunch for that much, hehe); (3) consistency was weird--not quite hot dog, not quite sausage, sort of like tamago? (egg/omelet sushi).
Frankly (har har) I like the dogs at Spike's Junkyard Dogs more, and those are much easier (and cheaper) for me to get, so I'll stick to them. I'm always up for a food-related adventure, though, and I do thank Chowhound for supplying lots and lots of very accurate info!
Speaking of: for anyone who is put off by the difficulty of getting to/finding Speed's, don't be. I was expecting a 40 days 40 nights in the desert scenario, but it was actually quite simple to get there (if not particularly scenic). From Copley I took the bus and walked; it was 30min. or so each way, and was exactly where Chowhound said it would be (thanks again guys!).
Speed's Hotdog Wagon
54 Newmarket Square, Boston, MA 02118