Trying to summarize the general opinion on Metcal on the forum (flame suit on):
* Performance is outstanding. Power delivery is huge despite apparently low power rating (40W/80W). No overshoot. It's as good as you can get, with some competing systems getting to that level (opinions vary).
* Ridiculously fast power delivery is nice but not needed is most standard cases. You'll see a difference vs. the FX888D for instance mostly on heat up time and soldering larger masses.
* The handle is very good. Your fingers are close to the tip and it doesn't get hot.
* The lack of temperature control is a big deal for some and a non-issue for others. One the one hand, some (including Dave) prefer temperature control without having a collection of tips for that purpose. OTOH most Metcal users use just one temperature tip (7 series for most, 6-series for some) and don't see the need to change (fast power delivery + no overshoot = no real need to modulate temperature to the work). Most Metcal users have a one or more different temp tip "just in case". Tips are very easy to change.
* Tip and power supply durability is extremely high. These are production line items and will last a long time. I have personally never seen a Metcal tip goes bad, but evidently it is possible.
* Power supply and tips are expensive if purchased new. Probably impossible to justify for a hobbyist or a low-usage environment. However, the whole ecosystem has been around for a long time and the market (EBay) is full of supplies and tips at substantial discounts. Many happy Metcal users on the forum have bought their system at very affordable prices on the second-hand market.
Thermaltronics is a good alternative for lower supply and tip prices, but if you're ok with buying used, you'll find many more Metcal items are reasonable prices.
If you're going Metcal, you might as well go with the 13MHz system (something like a MX-500 supply).
I would say that in the vast majority of cases, you won't _need_ a Metcal iron. But if you buy one (and are ok with the lack of temperature control), you will likely be very happy with it.
Disclaimer: I am a hobbyist happy with my Metcal. I have had a FX-888D was was happy with it too. I think the FX-888D could do everything I do with the Metcal. What I like with the Metcal is that you never have to worry about it, wait for it or tinker with it. It is one of those tools that allow you to focus on your work, not on the tool.
I have bought a number of tips and accessories at good prices on EBay.