Steven Frank has posted a comparison between the Sony Ericsson P800 and the T-Mobile Sidekick (AKA Danger HipTop). Steven has used the Sidekick as his primary phone for quite a while and recently picked up a P800 instead of upgrading to the color version of the sidekick.
One of the biggest surprises is that the P800 is actually smaller than the Sidekick. Don't believe it? He's got pictures.
Overall the P800 seems to have beaten the Sidekick, but since it is a much more expensive device, this is to be expected. T-Mobile really opened things up when they allowed anyone to add unlimited GPRS data to their plans for $20 a month (prior to that only the Sidekick's plan had unlimited data). That's why Steven got the P800 in the first place, and we're guessing a lot of other people are going to follow (maybe not witht the P800, but the lure of getting the Sidekick just for its unlimited data is now gone).