TreatMonk:
What is your opinion of the Conjuring spells and the Conjurer now? It seems to me you can do better with the demons and devils, at least it isn’t 1 minute casting time.
I'm not Treatmonk, but the Greater and Lesser Summoning Spells for Demons are interesting.
Lesser Summoning can give you a pair of Maw Demons that go on a rampage. That can be useful in limited circumstances, but a fireball will generally clear enemies out faster. This spell is nice because you can end it whenever you want, so "overkill" by the demons can be stopped whenever you want by ending concentration. Once they get that extra 30 hps, they're durable enough to be of some use, but using your concentration for two minor damage sources at that level is questionable...
Greater Summoning is more interesting to me and has already seen some play. Being able to summon Babau (for at will spells, including dispel magic), Barlgura (more spells), Vrocks (more risk, not that much better than a Barlgura if it even is), Shoosuva (for that paralysis and damage), and Glabrezu (not going to hold it in check long, but a lot of useful abilities).
Infernal Calling is still a 1 minute cast, so it is unlikely to be a combat spell for me.
I do not think that these spells make the Conjurer and better. The extra HPs are fairly meaningless and only happen late in the career. The concentration support is nice, but most wizards have pretty good concentration saves with feat or multi-class help (start fighter, etc...)
The real problem with these spells, however, is their competition.
At 3rd level you have fireball, fly, major image and several other great spells. Deciding to prepare this spell means you believe you're more likely to cast Lesser Demon than those spells - and that is hard to support.
At 4th level you're competing with Banishment/Polymorph/Improved Invisibility, etc... I couldn't justify preparing it until 10th level - and only because I was only preparing 1 5th level spell. My enchanter uses it to summon Barlgura (most of the time - love the -1 charisma save) and Babau, primarily, but only for limited purposes (Babau - Dispel Magic spamming, Fear spamming until enough enemies fail, Barlgura - Phantasmal Force or Entangle and then beating on enemies while I stay out of range).
If we get "better" demons in Mordenkainen's we might reevaluate, but right now these are tier 2 spells in levels 3 and 4 that you'd use conditionally, and they do nothing to make the conjurer more interesting.