What is OS 8.6 and do I need it?
OS 8.6 is this week's operating system from Apple - in fact it is really just an update of OS 8.5 (which is why there was such a hoo-ha when Apple originally said they were not going to make it available to third parties like Mac magazines to put on their cover CDs).
Whether or not you need it, is of course a tricky one. Each time you update to a new OS there are bound to be a whole load of conflicts and programs that refuse to work or work properly - the worst one seeming to be the Epson 740 driver problem outlined below.
The main benefits seem to be for users in networked environments, although the USB support is supposed to be better and the game sprockets (which help the iMac communicate with joysticks) are built-in to the OS.

Obtaining OS 8.6
As I say, there was a big stink when Apple said they were not going to make OS 8.6 available to third parties like Mac magazines to put on their cover CDs. You were supposed to spend time and money downloading it from the internet, praying that your connection didn't drop just before it finished downloading. Luckily Apple have seen sense and at least allowed MacUser to stick it to the cover of the 9th July 1999 UK issue. I suggest obtaining a copy of this rather than attempting an internet download.

Epson 740 printer conflicts with OS 8.6
If you have the Epson StylusColor 740 printer and want to update from OS 8.5 to OS 8.6 - beware! The USB printer driver on the UK Epson website causes a vicious conflict with OS 8.6 Make sure you have the firmware update installed, and then go to the US Epson web site at http://www.epson.com/support/styc740.html and download the file called MUSC745B.SEA.
See also Apple TIL 58407 which states:
Apple has released various firmware updates for the iMac and Power Macintosh G3 (Blue and White). Included in these updates are revisions for USB functionality. Check with the USB device manufacturer to see if they require a specific update be installed. If you are running Mac OS 8.6, it has the latest USB software updates though there still may be computer specific updates required. If you are using Mac OS 8.6, remove the following four Epson extensions and verify the extension named USBPrintDriver is installed and active in the Extensions Manager control panel. Remove these four extensions:
Epson USBPrintClass1
Epson USBPrintClass2
Epson USBPrintClass3
Epson USBPrintClassA
If you encounter issues with the Epson printing software, remove all the Epson printer software and reinstall the latest software version.

For reference, here are a few of Ben Nicholson's posts to the Apple iMac Support Forum:
ATTENTION: UK 8.6 updaters with Epson 740
Author: Ben Nicholson
Model: iMac rev.A, 128MB RAM
Category: Other
System S/W: Mac OS 8.6
Posted: Friday, 5/14/99 2:14 AM CDT
If you are using the UK version of the Mac OS, and you are also using an Epson Stylus Color 740 printer, and you want to upgrade to Mac OS 8.6, then the following info is for you. As I found yesterday (going for the world record for number of clean installs performed in one day), the printer drivers (software which allows your iMac to recognize the 740 printer) from Epson's UK website seem to be incompatible with version 8.6 of the Mac OS. The result for me was a frozen mouse pointer from start-up.
This 'freeze' is usually caused by a conflict between extensions in the Extensions folder; the printer drivers go into this folder, and the Epson UK drivers seem to conflict with some extensions (maybe the new USB-related ones) which are installed with the new Mac OS 8.6.
The solution that I found worked for me is as follows:
1.Download Epson drivers from Epson USA website (www.epson.com). Just follows the links (imaging products-printer products-downloads, etc), and you will come to a page with mostly Windows PC-related drivers on it. The one you want is near the bottom, and the compressed file that you should download is called "MUSC745B.SEA". In contrast, the offending drivers from Epson UK are in a compressed file called "SC740USB.SEA".
2.Insert OS 8.5 update CD-ROM, and restart, booting from this CD (hold down the 'C' key when you click on restart, and hold it down).
3.Perform clean install of OS 8.5. Restart.
4.Perform install of OS 8.6 update. Restart. 5.De-compress the downloaded file, MUSC745B.SEA, and follow installation instructions in the file called "Disk 1". Restart.
Hey Presto! Your iMac should now restart in OS 8.6, with no problems. If you now look in your Extensions folder, click once on the Epson driver extensions you've installed, select 'Get Info' from the File menu, and you should see that you've installed the following versions of the Epson drivers: SC740 (the one with the printer icon): version 5.54E Epson USBPrint Class A, 1, 2, 3: version 1.02 Epson Print Monitor3: version 1.23E
I hope this is of some use to other users; please advise feedback if this works, and especially if there are any continuing problems. In the meantime, I'm going to mail Epson UK to ask them to update their drivers. Best Regards, Ben - - - - - - - - - - - - -

Following my day-long saga of seemingly endless re-installation, I've finally found the right way to do it; in short, have a look at the reply from Apple Support to the related posts below. Follow their advice - go to the US Epson website (www.epson.com), and download the driver from there. You'll have to download a compressed file called "MUSC745B.SEA", which is about 4MB
I've now done this, and installed the driver on top of 8.6, and it works!!! Hooray!
Whatever you do, don't do the following:- 1) Don't install the Epson drivers from the CD-ROM. They don't work with the iMac. 2) Don't download and install the Epson drivers from the Epson UK website. This is the mistake I made; it seems that the drivers on the US site are the most up-to-date, while the UK ones seem to have caused me lots of headaches today. Good luck, and please advise if this works for you too. Best Regards, Ben Nicholson - - - - - - - - - - - - -

Subject: Epson UK advice on 740 drivers with 8.6
Author: Ben Nicholson
Model: iMac rev.A, 128MB RAM
Category: Printing
System S/W: Mac OS 8.6
Posted: Tuesday, 5/18/99 1:32 AM CDT
Further to the my recent postings to this site on the subject of seeming incompatibility between OS 8.6 and drivers for the Epson 740 printer (downloaded from Epson's UK site - the US ones work fine), I mailed Epson UK to find out what was going on. This is their reply: "We suggest that you install the driver then disable the print class one and print class two extensions from the extensions manager. This should stop the freeze." I hope this helps anyone still floundering about with this problem, as I was. I downloaded the drivers from Epson's US site, and they work fine for me, so I guess I'll stick with them. Best Regards, Ben Nicholson