Agree about Monarch - good god, don't do that!
Flight upgrades - depends on what you want really. We personally would never go anywhere long haul without them. I'm quite tall, well 6ft, and find the normal seats can be a bit uncomfortable.
Plus you get other benefits besides just a wider seat and more legroom. Being able to walk past all the people in the main cabin and check in at your own desk is worth it alone! lol
I can appreciate at book value they can be expensive. However, if you can get a discount off them it makes the decision a lot easier. But sometimes if you book late you can get discounts off the upgrades - we got £100pp off the upgrades for example on flights to Kenya in May.
If you find the normal cabin fine, then you probably don't need the upgrade - its down to personal choice at the end of the day. You can sometimes, if the flight isn't full, upgrade when you get on the plane, just ask the staff on the plane when you get on if there are any spaces in the cabin.
You have to pay, but its cheaper than if you booked it before.