I've had both the V3 and the V5 and I love them. I have the V3 in gunmetal and the V5 in stainless. Here's my pro's and cons.
Pros: Variable voltage/Variable Wattage. can change the power to suit your vape level or juice.
Easy button and menu system. Makes changing settings a breeze.
EGO and 510 threads, fits all common devices.
Removable top cap. Can be taken off for cleaning, or screwed up to the device to make a flush contact.
Removeable battery. Can replace the battery if the power is getting low.
Stainless Steel option - fingerprints are almost non existant.
Spring on bottom cap. fits a much wider variety of batteries.
Price. It is the best value per dollar IMO.
Cons: BIG. Not easily pocketable. Also makes a big thud if I put it down and accidentally knock it over.
15 Watt maximum. Like most Variable Voltage devices, it is limited to 15 watts, which limits it to primarily single coil
Angled at the top of the top cap. Just a beauty thing, but if it were strait, it would look better on more of my devices.
Too much gap between top cap and top of the 510 connector (vamo V5 only)
up and down buttons are too easily accidentally pressed. Sometimes vape, and realize the wattage has been kicked up or down some.
The competitors are the MVP 2.0 (no replaceable battery), Sigeli Zmax (price too high), Lambo 6.0 (Price+ no removable top cap), Provari (Price)
I prefer the V5 over the V3 for the most part. The V3 is heavier, looks funny, but seems to have a sturdier feel to it.
Just my two cents worth.