Lets talk mercury vapour bulbs.
I'm considering getting them but I am wondering a few questions.
1) Thoughts?
Depends on the situation really. They can be quite useful but can also be crappy.
2) Pro's?
Most of the reptile brands provide good UV light and heat in one bulb. They tend to last longer than tubes. They generally speaking cover larger basking sites then other bulbs.
3) Con's?
They are expensive. Difficult to disposal of responsibly. Depending on your needs may not produce enough heat at the safe UV distance.
4) How much UVA/B do they give out?
Generally the reptile brands are fairly similar in this regard and produce enough. But the non reptile brands are likely to provide nothing.
5) Do they get hot enough?
Good question, that depends on the species you keep. Most brand new MVBs have a safe UV distance of at least 30cm from bulb face to basking site. At this distance you could get enough heat for some species but most monitors are probably going to need more. Most would compensate this with a reduced distance, but as mentioned they will be getting into the higher than normal UV range. Aged bulbs might be more suited it that situation.
6) What brand to get?
As expensive as they are, if you are going to get a MVB get a reptile brand one.
7) Can they be used with on/off thermostats?
No but you can use timers
8) What wattage to hold a 40 degree hot spot (50cm away from where the hotspot is)
At that distance I would guess that you couldn't but as a cheat you could always move the basking site closer to the bulb. You could look into other solutions like a halogen and UV light or go down the metal halide path.
Cheers, Alex