Radagast was an interesting idea, but one which was implemented much better later in the game's life with Treebeard. Even before we saw what could have been Radagast didn't seem like a great deal. Now, I would say Radagast is more maligned than he deerves to be, but he's still not great. The thing is, he's a bit like the Master of Lore in that he's useful once he's in play, but justifying spending those resources on him rather than on something else is tough, especially in the early-game, which is when you want to play him to get maximum use out of his ability.
His stats are obviously not worth 5 resources. 2 is the best thing he has, and that's not worth 5 resources by any stretch. Meanwhile consider his ability. The healing option is almost never used, and in any case there aren't a lot of valid targets. The only Creatures who aren't Eagles are The Riddermark's Finest and Bill the Pony, neither of which are exactly a priority for healing; meanwhile the Eagles are mostly low and expecting to leave play, with the only tougher ones being Eagles of the Misty Mountains, Landroval and Gwaihir. Once in a blue moon, or if you're playing Journey to Rhosgobel and stack up enough resources to heal a meaningful amount from Wilyador the healing might by useful, but by and large you use Radagast's resources to play cards. Once again, non-Eagle Creatures are very rare so his main use is for Eagles. And certainly there's utility in that, helping to get more Eagles out quicker - but it's one extra resource per round, so it'll take 5 rounds just to break even on what you'd have if you paid for Eagles with the resources you actually spent on Radagast.
That's the standard line of argument. I feel it misses something in that it only considers Radagast's stats and his ability separately. Clearly neither stats nor ability are worth the steep cost of 5, but what about the two together? If we say Radagast's 2 is worth 2-3 resources, then by that logic it only takes 2-3 rounds for Radagast to break even by paying for Eagles, which is certainly more reasonable. This makes a decent case for Radagast being usable in some circumstances. The problem remains, however, that he costs almost two rounds' worth of resources to play, and you might really want to play other cards with those resources to ensure you survive the early rounds. So by the time you feel you can get away with saving up for Radagast, it may be too late in the game for him to make that much of a difference. Not to mention that if you're playing an Eagle deck, they're a very combat-focused archetype, so the 2 may not be something you particularly want that much. Now there could be a case where you play Radagast with resources from a sphere other than , so playing Radagast doesn't reduce the number of Eagles you can immediately play - but then the reason to play Radagast is if Eagles are your primary focus whereas this hypothetical seems to imply Eagles are not so central to your strategy or you wouldn't be getting so many non- resources. And if you have 5 spare non- resources there will probably again be better ways to spend them than Radagast, especially if Eagles are not the main theme of your deck.
Now there are a couple of other benefits to Radagast in that he is an Istari. That means he can use a Wizard Pipe or a Word of Command. Or Flame of Anor if you want, but if your Eagle deck needs that kind of extra combat boost something may have gone wrong. Word of Command obviously is incredibly useful, and if Radagast's main purpose is to help play Eagles then losing his action shouldn't be too big a deal. Wizard Pipe can be useful if you're built around it in other ways, with cards like Expert Treasure-hunter (plus Hidden Cache perhaps) or of course the classic Vilya. These are some interesting and potentially powerful options, but they're a fair bit of work to be the only things you're using Radagast for, so I would be more inclined to view them as potential bonuses to an Eagle deck using Radagast.
So while there is some definite use to be gotten out of Radagast, potentially even commensurate to his cost in the long run, he's going to be much more palatable if you have some means of getting him into play cheaply. Of course any cost reduction we use for Radagast we're then not using for other things, but since Radagast can justify himself once he is in play, the issue is not so much making him worth it, but making him a decent early-game option by allowing us to play him as well as other cards that have more immediate benefits at that point. I mentioned Vilya above, and in addition to having synergy with the Wizard Pipe option obviously it can also get Radagast into play for free. Elf-stone, Timely Aid and Send for Aid could all work, and Gríma would at least drop him to 4-cost, though since that's still more than a round's worth of resources it continues to be hard to justify. Vilya with Elf-stones as a backup option may be the best choice, since of course Elrond can pay for allies of any sphere so you don't lose out on your Eagle focus.
In the end, Radagast is interesting and can pay off in the long term, but just too expensive in the short term to be worth it the majority of the time. He suffers from the general over-costing of some cards in the first cycle when the designers were still getting used to how to balance things; or maybe they just wanted him to cost 5 to match Gandalf but didn't balance out his stats/ability to equal that imposed cost. Either way he's mostly left in the dust unless you're in a context where you can afford to take things slowly at the start or you can get him into play for less than his actual cost.