Before I say anything about its LA: I want to make it clear that the grimweird is a very interesting monster that I like a lot. The idea of a summon-spamming, hell-annoying, frail corpse is quite neat, if only I say so myself.
Their chassis is, naturally, weak. 12 undead RHD, low stats (20 charisma is the highest, and not particularly impressive), absolutely awful strength (better hope you got a party mule), and two natural claws that are only mildly interesting because of the negative level they inflict.
The most notable special ability is Unholy Grace, which add's the grimweird's charisma bonus to its saving throws and AC. This is quite an useful ability, as I'm sure everybody will agree.
Finally, there's the grimweird's real selling point: the SLAs. Summon Monster VI every 5 rounds, with an initial duration of 12 rounds, means that at any point you can have two reasonably powerful summons at your beck and call.
The most obvious point of comparison for these guys, in my opinion, would be the Star Spawn Binder, who gets the similar ability to summon monsters every half-minute. Obviously those abilities aren't equal: the binder initially has access to less powerful summons (though it gets stronger ones from level 14 on), has different limitations on what it can call (only evil creatures vs. pseudonatural ones), and the binder will have an easier time raising their caster level.
Furthermore, the grimweird also gets 1/5 rounds standard-action Lesser Planar Binding. However, without Magic Circle or Dimensional Anchor, and the hatred of every lower planes being in existence, I strongly doubt this ability will be very useful. That said, with a reasonable UMD score and a wand, this SLA can quickly turn very powerful. Because of the general shenanigans that result from summoning a fiend every thirty seconds, I'll add an asterisk.
However, considering everything I think that at +0* LA the grimweird can make a fine tier 3 character, at least. Feel free to discuss.