3PL experiences

I found a handy list of third party logistics providers, some claiming to specialize in Amazon fba/fbm. Now I’m in the research phase and would like to hear about anyone using one for FBA specifically but FBM would be helpful too. My fba prep looks good but I can’t start the relationship committing to 15,000 units right off the bat.

I’m especially looking for experiences with a 3PL where you aren’t doing insane unit volume and/or it’s sporadic, a lot now and maybe a dry spell for a bit, then more again.

Many have nice looking websites but I’m really looking for user experiences from this community. Also any info on how costs actually came out relative to units moved, volume of units, and revenue.