If you target a land area with the spell, it will reveal the production/grain/essence values of the tiles affected.  Assuming those tiles have anything to reveal.

Best spots are usually closest to mana nodes, at least 5 squares away from a city

OK, if you guys are talking about the Birth Of Summer/Bloom Of Twilight spell, I've used this successfully on 'desert' squares.  what it does is assign resource values to squares that currently do not have any, which allows pioneers to settle them. 

This seems to work best when targetted near resources (crystal crags and such), as proximity to resources seems to affect the values (food, material, essence).

The first couple of times I used this spell (during the beta), I was trying to turn dead forest into live forest near one of my cities (so that I could clear it for structures) but the spell isn't meant for that purpose apparently.  Reading the spell .xml though, it is supposed to work on forest as well, so I don't know what to think about that.

Worked on forest tiles for me as well.  I was able to use it to create a pretty sweet new city area that become one of my most powerful cities, so yeah, it can be very useful.  I doubt it would work on already useful land, so look for the wasteland type areas and use it on those.

This. Play around with it a few times. Once you "get" where it is likely to create tile yields, the spell can win you the game.

I've created 3 essence tiles with it by casting with both a crystal crag and a shard in the area of effect. 

Great spell, especially now that it is a tier 2 and cheaper to cast.

well that makes sense while still having a hugely inadequate spell description.

Perhaps most of you guys doing it wrong.

It won't work on desert.

It only work on wasteland (the original buildable land that is now unbuildable due to the cataclysm from the war with the Titans).

 edit:

To know if it's a desert or a wasteland, just hover your cursor to that tile, if it says desert then it's not a wasteland, otherwise it's a wasteland.

OK, color me into not seeing the difference between desert and wasteland camp.  In my current game, I tried casting this spell twice in succession, with no change.  Turns out I was casting in desert.  I thought the spell was partially broken...

Really, though, even in desert it should do something, even if it's a 1/1/0 proposition, just to let you know that you haven't chosen an ideal spot, rather than just not having anything happen and leave us all wondering if the spell is broke.

This also applies to those olive drab colored scurvy trees.  You'd think Revive The Land would work on those, but it doesn't.  It does work on the twig trees though, and those also regrow when your city's ZOC expands.

That's what the spell does though, changing wasteland only. No desert, ocean, fissure, etc. It always works for me, since FE v1.00.

The description of the spell doesn't have much detail. But if you pay attention to the lore (which 99% of folks doesn't) you can easily guess what that spell does. Just watching the opening movie once is sufficient.

Generally if you want high essence tile, you need to cast it near a fertile land, i mean grassy one or living forest, not the dead forest. By dead i mean dead forest, not purplish fallen forest.

Some example: in an 9x9 (81 squares total) wasteland, if you cast the first 3x3 (9 squares total) revive land on the middle 3x3 squares of that 9x9 squares, you will have 3x3 tiles with bad essence yield, after you turn all 9x9 to a buildable land (land with yields), the first 3x3 land (especially the middle one) you turn to life will increase its essence yield.

Casting on a 3x3 tile which all of 9 squares already have yield (meaning all 9 squares are buildable) will do nothing. However if the spell is cast on adjacent no yield tile (non buildable tile) to said 3x3 tile, the yield of all 9 squares will change, mostly for the better, and especially the middle one of the 3x3 tiles.

Wasteland near a dead forest or even the dead forest itself will yield better production than open land (open land will become grassland if revived which mostly have high food yield. Wasteland is no yield tile / non buildable tile (not desert, not ocean, not fissure though).

Casting it in your own territory (outposts preferably, it's a waste if too close to existing city) makes tile yields the most likely to appear, in my experience.

Whenever I have cast on plains or forest tiles, my territory or not, I have seen at least 3 city spots appear. 

No tile yields can appear within 6 tiles of an existing city. 

It does what your cities do as their ZoC expands, it turns the dead lands into green/purple.

If you use it somewhere far enough from existing cities, it MIGHT reveal tiles for cities, but it is not mandatory of it.

I tend to use it beside forests and rivers and I have gotten many cities with it. Specially useful when there is a massive wasteland between my empire and the next one, so I use the spell near their border with a pioneer waiting, and if I get lucky, I get a beachhead to work from.

Just try to consider location, where would nice, settle-able land be? By the deserts? Or by the mountains, coasts, rivers, and forests? It tend to follow the same logic as the normal city spots. But in the end it IS a hit or miss spell, can be a game changer if you get a good spot, but can also be a waste of mana.