Really depends on what you're doing and what you can afford.
If you're Foursquare and you can afford to double the cost of your infrastructure because you want some of the benefits of the AWS platform (and there are plenty of benefits these days) - then it's tremendous to say the least. Amazon is doing really incredible things with AWS.
If you're doing less than a million uniques per day, you can go get three tremendous machines, lasso them together, with a web server + main db + slave db, for between $800 and $1250. You can get 100tb of bandwidth on a 1gbps port (Amazon gives you none), dual 5645 Intel processors (or 2x16 core AMD), 48gb of ram on the db machines (96gb if you want to pay another $200 / month), with a RAID 10 config SAS 15k drives. That equivalent setup with Amazon would cost you $5k to $10k depending on what you config. You can get this setup from reputable hosts like WebNX and SecuredServers; if you want to pay more for a better host and get a little less, you can go with Rackspace or Gigenet or Softlayer.
If you're Foursquare and you can afford to double the cost of your infrastructure because you want some of the benefits of the AWS platform (and there are plenty of benefits these days) - then it's tremendous to say the least. Amazon is doing really incredible things with AWS.
If you're doing less than a million uniques per day, you can go get three tremendous machines, lasso them together, with a web server + main db + slave db, for between $800 and $1250. You can get 100tb of bandwidth on a 1gbps port (Amazon gives you none), dual 5645 Intel processors (or 2x16 core AMD), 48gb of ram on the db machines (96gb if you want to pay another $200 / month), with a RAID 10 config SAS 15k drives. That equivalent setup with Amazon would cost you $5k to $10k depending on what you config. You can get this setup from reputable hosts like WebNX and SecuredServers; if you want to pay more for a better host and get a little less, you can go with Rackspace or Gigenet or Softlayer.