They are safe to use and offer the most realistic lighting for reef tanks. The only cons about the lamps is that they run hot which can raise tank temp (possibly need to invest in a chiller ~$400), and they use a lot more electricity. They will maintain SPS (high light) corals and high light clams, but if you want them to grow, you have to get actinic lights. A MH lighting fixture can run around $200-300, whereas a MH + T5 actinics fixture are around $500.
All corals don't require the same amount of lighting, so make sure you focus on how many watts per gallon is needed when purchasing certain corals.
high light (SPS; clams) = 5-8wpg
med light = 3-5wpg
low light = <3wpg
You can keep several corals with different lighting requirements in your tank as long as you place them correctly; such as high light corals on top, and low light on the bottom.
If the tank you have is 34 gallons, you can get away with having 1 250w MH, but make sure it's high enough or it will cook/bleach all of your corals. How deep is your tank (water surface to sandbed)? The size of the bulb may only need to be 150w if you have a deep sand bed.