Hey, who is going first ? 
Some small models like 40C are not support.
Just have a quick look at release notes, there is a loooooot of know issues...
2 Solutions
I have two 500Ds. One for production, other is for testing. I swap them back and forth so I can fall back quickly during upgrades or an outage. I highly recommend this model if you do not require a HA config as it makes testing much easier and less error prone. I turned up a new 1Gb connection and decided to use 5.4 after doing a lot of testing. I realize based on past experiences this is a risk, but I've been using FG for 11 years and this build appears way more stable than previous ones.
My production 500D was running 5.2.3. Had a problem with EC cert that after I installed it, the cert GUI went away. I copied this config to my second 500D and upgraded to 5.2.6 and confirmed that would fixed the issue. Took some time reading and understanding changes in 5.4. We do not use VDOMs or HA. Fairly standard config with one LAN, one WAN, about 40 rules and 500 address definitions, multiple security policies. We do use some identity based policies and are working to get a FAC200D up and running to enable two factor auth for VPN. After this is stable for a few weeks, I'm going to build a new config using WANLLB incorporating our backup 100Mbps connection, but didn't want to complicate issues during the main cutover.
After I had 5.2.6 running, I made sure to review "diag debug config-error-log read" to make sure no major settings were faulting after the upgrade. It is critical you use this to know what settings in your old config log didn't port correctly during an upgrade. I was also careful to make before and after backups of my configs when upgrading.
After 5.2.6 appeared stable, I upgraded to 5.4. I had already spent a month playing with a factory default so I could be familiar with the options. The upgrade occurred without problems. I did decide to wipe and rebuild both my SSL and IPsec VPN configs to ensure there were no legacy options config issues. One thing I have found is that using the CLI "show" command when reviewing configs is not as good as "show full-configuration" as it displays all of the options. Some of the defaults can lead to problems you don't realize are there without the full command.
The GUI is very 8 bit, but isn't everything these days
. I really like the following:
[ul]
Overall the new GUI is working well and we are getting ~850Mbps throughput with all preventative measures enabled. I've seen some others mention that memory util goes high. Ours is sticking at 30% so that is good. Tomorrow will be the test when 125 users start running traffic through it. I have my second 500D running 5.2.6 ready to fall back on it if issues arise with 5.4 but so far so good.
I think 5.4 is a very good release, but be careful if you have a complicated config or only a single unit to test on. Read the 5.4 Admin Guide regarding Firmware Updates. Have local copies of your config and current firmware so you can fall back to it if problems occur. I would suggest that you simply reload current firmware, then factory reset, then reload your most recent pre-5.4 configuration backup to ensure the downgrade process works. The revision feature in 5.2 and 5.4 is very nice to have during this process, but I always keep local config copies also.
I have heard that 5.4.1 will be out later this month so I look forward to testing that when it is released.
