I've read many times now from players that are avoiding the Priest upgrade because all it does is feed your opponent gold and XP. Essentially players have overblown the costs of this upgrade and have made buying creep upgrades a bad idea. That's not an accurate assessment of the upgrade.
Priests aren't always a good or bad upgrade to get. They just shouldn't be your first priority. When buying upgrades you really need to assess how you are doing against your opponent. Consider the map and game mode and buy accordingly. In some cases Priests are a great purchase. In other cases Priests should be low enough on your priority list that you won't be buying them.
On the plus side, Priests allow each wave to deal more damage to towers. They also allow each wave to last much longer before getting wiped out, unless they go up against demigod AoE abilities. Left alone, Priests make a huge difference in the effectiveness of your creeps. Priests also heal allied demigods at regular intervals, extending the time your demigod can stay on the front lines without retreating. This isn't something to underestimate. The key to outlevelling your opponent is to stay in your lane as much as possible and not retreat back to your crystal. Using Priests to do this can help win the game.
In general you're in good shape with Priests as long as you've got multiple creep waves going and you're trying to wear down your opponent's towers. Maps like Leviathan, Mandala, Zikurat tend to be good, because you can get creep waves that are unopposed by enemy demigods. When you're in control of the map and you have more spawns than your opponent, Priests are awesome. Even if you don't have more spawns than your opponent, Priests are very helpful if you leverage them to take down enemy towers. They are extra bodies that the AI will shoot before your demigod, and they keep your health up while you stay on the attack.
The downside to a Priest purchase is that you're spending money for the upgrade that you don't put into something else. And if you're not advancing your map control with your creeps then it's wasted money. Plus there's the possibility that your opponent is killing more creeps than are dieing to towers. And if you buy Priests you really want to invest in the Attack and Defence upgrades for creeps. That starts to add up.
You're not likely to get much benefit from a tight game on Prison or Crucible from your Priests. Those maps have tight lanes that are easily farmed. Creeps rarely make it to your enemy's towers unopposed, and most players end up dueling it out in the same space. This is where players who cry about giving your opponent XP and gold have a point. That's especially a problem if you're losing map control.
Cataract and The Brothers are maps where Priest MIGHT help against towers, but can also help out if you steal one of your opponent's portals. Those aren't worth taking unless you have upgraded your creeps, but can be devastating when you have powerful creeps. But they are also maps where you can easily get your creeps farmed if you're not supporting them properly. So if you're being bottled up by your opponent or facing good AoE demigods, go spend your money on Defensive Upgrades or items.
Overall Priests aren't nearly as bad as some people have been claiming. If you support them with your demigod and maintain portal control Priests are a great investment. But if you're not doing well, on a map with tight lanes, or against good AoE powered demigods, you can think about staying away. Overall you should use your brain and assess the whole situation. Overall the statement that 'Priests are just free XP and gold for your opponent' only applies to players who don't know what the hell they are doing.
