Usability 
Mobango is a very popular mobile file sharing site. After registering at the PC site mobango.com, you can upload ringtones, wallpapers, videos and java apps from your PC to Mobango and them download then to your phone from the Mobango mobile site wap.mobango.com. When you upload a file you can flag it as either public or private. Any register user can download public file which currently number over 200,000. The file size limit is a relatively generous 10 MB per file. There is minimal help on Mobango's PC site and none on the mobile site. The site is pretty easy to figure out though. Here are a few tips, you can't immediately download the files you have uploaded. Uploaded files end up inyour "My Mobile Files" area on the PC site and you have to transfer them to your "Locker" before they can be downloaded. You can also browse and search files on the PC site and send any you find to your locker.
Mobango has a social aspect too, you can search for users by age, sex, country and make and model of phone. Members can exchange messages and files through both the PC and mobile Mobango sites.
Most Mobango services are free. I did get prompted to buy credits when I tried to add a 2.5 MB video to my locker. I could buy various quantities of credit from $2 up. There was no explanation how much credit was needed to transfer the file in question. I had no trouble transfering files up to 2MB in size and latter after I uploaded more files I was able to move the 2.5 MB file to my locker for free. So maybe you can earn credits by uploading.
Mobango can convert some content to mobile format. There is a simple online image editor which allows you to resize crop and add effects to uploaded images. The site also claims to be able to convert non-mobile video formats to .3gp. I tried converting a 2 MB .avi but conversion failed. Mobango claimed that the original file was corrupt even though it played without error in the embedded player on Mobang's PC site.
All sites hosting user submitted content have a problem with users sharing copyrighted material and Mobango is no exception. Each time a user uploads a file Mobango requires the user to check a box asserting that the file is their's to share legally. I'm sure they respond to take down orders as well. Manually checking 200,000 files is impossible and even YouTube hasn't ye been able to do programmatically. I don't know what the answer is to intellectual property issues on file sharing sites. Sharing content you have created is great, sharing the hard work and livelihood of musicians and software developers is not. I hate DRM and the heavy handed approach of the RIAA, etc but just asking people not to post or download copyrighted material isn't going to work. What do you think? Comments please.
For more on Mobango, see Oliver's review at MobileCrunch.