I really like the concept of Forum (but I haven't played with it yet). I'm a bit of a sucker for this type of card though. The two main cards this has been compared to are Warehouse and Cartographer. I'd argue that Forum combines the best aspects of these 2 cards while simultaneously eliminating most of their weaknesses.
Forum is vastly superior to Warehouse because it doesn't reduce your hand size. The dream of Warehouse is "Oh yeah, it lets me skip all my junk cards -- I'll get some of those!". Then you have a turn where you start by playing 3 Warehouses, and suddenly you are out of cards in your hand. Forum doesn't have that issue -- you can play as many as you want, and you'll end up with a starting hand of your deck's 5 best cards.
As a $5 cantrip sifter, Cartographer is the closest previous card to Forum. The big difference between the two is that while Cartographer sifts from your deck, Forum sifts from your entire hand + top 3 of your deck. This is a huge advantage for Forum. Even if you haven't drawn any cards yet, that gives Forum a search space of 7 cards to sift through compared to the 4 of Cartographer. If you've already drawn cards, the search space is even larger.
A common situation I've run into is a starting hand with one Cartographer as the only draw card. You play the Cartographer, and it draws something boring while revealing a couple cards that you really wanted this turn. Forum eliminates this issue because it draws everything before it sifts. Sure, you can't improve your next turn this way, but Forum is way better for your current turn.
Finally, we have the below-the-line "free buy" aspect of this card. This is another huge advantage of Forum over Cartographer. One of Cartographer's biggest faults is that it costs $5, and you just can't afford to get it over the alternatives. On the other hand, it's really common to overpay to get an important $5 card when you don't have a +Buy. I think it will be fairly common with $7-8 to skip a slightly better $5 card and pick up a Forum plus a cheap engine component. Even if you already have +Buy in your deck, you can't always line it up with your money while you're building. Basically when +Buy is limited, this card allows you to pick up extra components while building, and it gives you a huge boost in consistency during the end-game if you can afford Forum + Province a couple times.
As others have mentioned, Forum is sneakily good. It's definitely not going to challenge the top $5 cards, but it's basically always going to be a good card if you have any junk in your deck at all. It combines the strengths of Warehouse and Cartographer while eliminating their main weaknesses.