Mac App Store allows downloading macOS 10.12, Sierra, on unsupported Mac Does the Mac App Store check for macOS 10.12 hardware compatibility before downloading?

According to what I've been reading on the web, my MacBookPro 5,5 (mid 2009) should not support macOS 10.12, Sierra. However the Mac App Store allows downloading it. 

I would like to prevent downloading 5GB of files and finally discover that the installer says "your mac is not supported".
Officially your MacBook Pro (13" Mid-2009) is not supported by macOS Sierra. However, for whatever reason, the option to upgrade to macOS Sierra is appearing in the App Store for a whole range of unsupported Macs. In fact, today I saw this occur on a MacBook Pro (15" Late-2008) model that was running Snow Leopard 10.6.8, even though Apple doesn't officially support upgrading from this hardware or OS.

As to whether you can install it, the answer is yes, even though this isn't officially supported by Apple. If the App Store allows you to download the upgrade, then you could just download it (without installing it) and then make a copy of the installer. Once you've got a copy you could try installing from the original downloaded location to see what happens. If this doesn't work you can download the macOS Sierra Patcher Tool for Unsupported Macs. Make sure you read all the information on this page as it contains important information you should weigh up before proceeding. If it was me I would not proceed without a full backup ready so you could recover your system if all goes bad.

In the end, whether you should download the upgrade is up to you. Do you want macOS Sierra? Many of its new features are not supported even on Macs that are officially supported by the upgrade - so the answer to this will depend on what appeals to you in upgrading to macOS Sierra? To help you decide, go to Upgrade to macOS Sierra and scroll down to the Feature Requirements section.  

If what appeals to you isn't likely to work because it's only supported on much newer Mac models, then I wouldn't bother.