A lot of people here tend to "sell" the N900 with openness, customization, linux OS, neglecting to mention that this really is a well-rounded and full-featured consumer product. I'm not a linux geek or a programmer (although I am a geek in general and I'm not afraid to delve deep if necessary), so I can understand if these arguments alone aren't convincing enough for someone.

Here are some of my reasons for preferring N900:
- Great integration of IM and VOIP into the OS, expandable by plugins
- Great integration of content sharing, expandable by plugins
- Varied and expandable codec support - can play pretty much anything I put on it
- Built it media streaming over home network (DLNA), internet radio streaming
- Best mobile browser which shows the real internet, including full flash
- Great connectivity - 3G HSPA, WiFi, BT, FM receiver + transmitter, IR(!)
- Very high pixel density display - 800x480 on 3.5"
- Awesome camera in good lighting, average for a cellphone in darkness. Shoots high res 800x480 video
- Great conversations app with threaded IM and SMS (although not merged)
- Lots of storage - 32GB built in, microSD slot for expansion, mass-storage mode supported
- TV out
- Powerful and flexible platform which allows a lot of freedom, from OS plugins and extensions to overclocking

Of course, there are some cons:
- Relatively basic phone functionality - no voice dialing is the one that bothers me most
- Relatively basic organization functionality, calendar lacking in features and has problems syncing to Google Cal via exchange
- Email client is pretty basic and clumsy
- Practically nonexistent Ovi / PC Suite support (may be coming soon, there is a registration for a new beta version in May, specifically targeted at N900 owners)
- Low volume of available software, in particular commercial software. This is mostly due to Nokia not having Ovi Store completely ready for the device yet, and due to the fact this is a niche device after all. There is some user created software, some is very high quality and most is very useful, but what is severely lacking, that is probably too much for the community, is an office suite and navigation (alternatives to Ovi Maps and Sygic)
- Device works almost entirely in landscape.
- Some reports of build quality issues, especially the USB port. I had no issues so far.
- Could use a bigger battery. Nokia made an odd choice of using the 1320mAh battery from the 5800, a much less HW intensive device.

Hope this brief summary helps you decide. Unfortunately I've never had an Android device, so I can't help you compare them directly.