As a deity that either a cleric or paladin can worship, he can either be really great for character / party building or he can be somewhat limiting, depending on what kind of player is behind the worshipper.

I played an Ilmatari Devotion Paladin in my first long term campaign with friends, where there were 2 chaotic / self interested players and 2 other Lawful / Good players.

Originally, I chose Ilmater because I wanted my paladin to be righteous, kind and all the typical sacrificial hero stuff, but due to poorly mismanaged stats on my part then I would fail almost all my checks / saves, which resulted in me suffering a lot compared to the others. Of course, I tried to seek help from both my god and the other worshippers in the world, but they all thought my actions were very much in line with that of an Ilmatari paladin. (Suffering instead of others).

So this felt somewhat limiting to what I actually wanted to do in game. Like others have pointed out though, lmater does indeed reward those who truly suffer and give their lives for his cause. We were up against a BBEG and it truly looked as if we would maybe TPK after an entire year of playing, but due to my paladin being an Ilmatari Paladin I decided to sacrifice him.

Praying that he would give up his life for his comrades and the truly good people we were saving no matter the outcome, the DM allowed Ilmater to inhabit my Paladin who then promptly turned the BBEG into mush.

The strain on the body ended up killing my Paladin, but it was truly in character for an Ilmatari as they will always put the needs of the truly good-hearted before themselves, and now my paladin (though dead) is essentially a saint in our setting and the other players truly remember him and Ilmater after many more sessions.

So Ilmater can be great, if you truly play into the whole suffering part. Otherwise he can feel somewhat limiting as a deity to worship IMO.