Hiya!
*shrug* I wasn't going to comment, but seeing as this thread is getting a lot of back and fourth between opposing views, I'll toss in my 2¢.
I like the
idea of PrC's. I liked the
idea behind them even back in 3e. Back in 3e, they
were specifically optional to the point of not even being included in the PHB. They were purely a DM-creation thing. They had, what three example PrC's? Two maybe? Anyway, if you read the description of just what a PrC was
supposed to be... and then look at what they
actually became. It's like night and day! It went from "
A DM's personally created extra stuff for players, created specifically for the DM's personal campaign", to "
Wowzers! Give us money! Look at all the kewl powerz you can get with these 27 new PrC's! And there's more where that came from! Just wait two weeks and we'll have another 20+ PrC's...and those ones will be even moar kewl!!11Q1k23221`11!!!"
I think that's what annoys me most about them. They had such great potential to enhance a DM's campaign and really make it unique. But instead of providing the tools for a DM to create his own, they instead opted for the 3e party line of "Why teach them, when we can just sell to them every day?". For business...good...for RPG community...BAD. Nothing, and I mean
NOTHING gave me more head aches, more negative feelings, more "why am I doing this again...it's not even fun anymore!" thoughts than the
endless arguing with new try-out players (or folks I met in my FLGS and just started talking RPGs). I'm sure I got called (sometimes to my face) everything under the book with regards to being a "bad DM". The
"You just don't want us to have fun", to
"You're an idiot if you can't handle one little PrC!", to the ever-popular
"Oh, you must be one of those old school killer-DM's who can't handle when PC's can actually mess up your plans to kill them". 
The attitude fostered by the "We're players. There's more of us than you. YOU have an OBLIGATION to make the game fun for US! And you have to do it
NOW!" crowed that developed, IMHO, because of all the splat-books and one-upmanship.
Looking at this first Prestige Class installment... I'm a bit worried. They seem to be basically just taking a 3.x/PF PrC and "converting" it to 5e rules. The only thing that gives me hope is that it at least mentions the DM's involvement in that the DM decides and the DM chooses if and how a PC can gain a PrC. It's not just a matter of "Bing! I gained a level! I'll take this Feat, and now have the prerequisites for taking the Mystic-Shadow-Ninja-Death-Monk PrC!"... now the PC has to actually
find an NPC to 'teach him' the ropes (or whatever the DM sets up as appropriate for his campaign). Its completely in the hands of the DM how easy or hard this will be.
That said, I'm sure the complains about "sucky DM's" will just morph into "
Don't bother...he never lets us take PrC's! Not without first having to stop what we're doing, travel to some far off land, climb a mountain inhabited by savage humanoids, reach the summit, enter the Cave of Trials, and defeat the Shinning Serpent of the Clouds! Man... I just want to take this PrC and get this special kewl new ability that nobody else has and that no bad guy can oppose! ...*grumble grumble*... ".
Lastly, why? Seriously. Why introduce PrC's anyway? They are
completely unneeded in 5e. Do it with backgrounds and Archtypes. There's no need to "compartmentalize" new campaign stuff (rules, weapons, spells, etc) into something that is only available to a single "Prestige Class". The Rune stuff would have been
MUCH better served as an optional spellcasting 'style'. Then create a trio (just random thoughts here) of classes...say, Warlock, Ranger and Cleric... and give them each an Archtype that uses the new "Rune Magic" stuff. No new "class" needed. By having Rune Magic as a new optional addition to the game, it keeps it FIRMLY based in the "as campaign needs" barrel and not the "as player wants" barrel. IME, this sort of approach is significantly better for campaign continuity and creativity. One DM may have a unique theives guild of dwarves that use secret Rune Magic to infiltrate and spy on their enemies. Another DM may have Rune magic as the ONLY type of arcane magic available. A third DM could have minor Rune Magic being something that anyone can learn, and many common folk use simple runes to enhance their day to day lives (ala the earlier Runequest RPG). But making a whole new form of magic, and then putting it into a PrC just makes more work for DM's who now have to strip out things they
don't want, as opposed to simply adding in things they
do want.
As I've always said...it's easier to give players stuff than to try and take it away from them.
"Here, the northers tribes use Rune Magic. Here are some archtypes you can choose if you are a Northman/woman" is a lot easier than
"Here is a list of PrC's that are banned". See which approach makes your players more excited to play.
^_^
Paul L. Ming