I have a heat pump. General trade offs are gas vs electric and heat maintenance (i.e. Keeping a set temp for several days vs a quick warmup for a couple hours).
In my case, I don't have a spa. I just use it to extend the swim season (esp since I'm a baby in cold water). With a 7 mm solar blanket, my cost was roughly $50 per month in electricity for my climate/set temp combo in March.
If you had a spa, I think it will heat over 100, but you really need to maintain that temp and get a good cover. Not sure if people with a spa frequently use a heat pump.
Part of the heat maintenance thing is that it takes a long time to get up to set temp for me, about 1-2 degrees per hour) and takes more electricity while doing that as opposed to getting it warmed up one time and just maintaining it for 1-2 hours per day.
They also make heat pumps that cool as well. For a few hundred dollars more, I wish I would have gotten that one as my pool can be bath-like in the summer.
Finally, heat pump doesn't work well when it's cold outside. I live in Austin, and it works well for this climate. My house also uses a heat pump. If I would have planned ahead, I wonder if I could have done a geothermal heat pump system servicing all three units - that would have been cool.