Must have Plugin.
Mainly because of Oraxen and MythicMobs Support (for me)
It gives totally new ways to handle with these Plugins and gives the possibilitie to make a new way to create dungeons.
One could be "Place the head" Player breaks the head in the generated dungeons and summons with it a Boss or a Mystic Deer which you can if Boss kill or if a Mystic Deer tame.
So there are planty cool options and it works very good.
Nice Developer too and gives fast response.

This review will be posted on both MythicTotem and BuildableMobs' pages, I decided to compare and contrast the two since they are both very similar:

1: entity support:
BuildableMobs: Minecraft, ItemsAdder, and MythicMobs
MythicTotem: Mythicmobs only (but can execute custom commands so you can probably spawn mobs from IA and vanilla too)

2: Custom Block support:
BuildableMobs: CustomItems, Oraxen, ItemsAdder
MythicTotem: No info on the wiki but it does appear to support ItemsSdder namespaces and block

3: Features:
BuildableMobs: 3d totems, you can build and add stuctures in-game, in-game GUI editor, placeholders, can execute custom commands and other actions besides summoning mobs, allows you to configure how the totem can be activated (a player must place the final block, or it must be activated by a piston, or maybe block physics, or interacting with a block on the totem, etc.).
MythicTotem: 3d totems (making more than 3 requires paid version), placeholders, can execute custom commands and other actions besides summoning mobs, pricing system (paid version only) that lets players have to "pay" with either currency or xp or an item(s) to activate a totem and it has support for many custom item plugins.

4: Price:
Buildable Mobs: 5 GBP or about $6.35 USD
MythicTotem: Limited free version, $4.00 USD paid version

5: overall thoughts:
Both are good, I personally Prefer BuildableMobs, as it's much easier to use, has a better wiki and some nice features, such as fully in-game creation of spawning totems without having to edit any config files, that being said, MythicTotem is still a great free option, and the paid version has the price system for totems, which BuildableMobs does not have, and some servers might want to make use of. Ultimately both are great options and it depends on what you need for your server.