It's certainly swingy, as it might group 4 Silvers or it might do nothing at all. But discard pile searchers are always going to be swingy (unless there's a built-in way to fill the discard pile or to otherwise ensure that it only gets played when there is stuff in it), so let's accept that fact and analyze its power.
First, what is the average number of Silvers this card is likely to retrieve? Well, the average case is that you get this when you're in the middle of your shuffle. (Let's be charitable and assume no drawing cards.) That means (n-5)/2 cards are in your discard pile, (n-5)/2 cards are in your deck, and 5 cards are in your hand. To figure out what percentage of our total cards are in the discard pile, we have to know what "n" is. Let's say n=15. In that case, one third of your deck will be in the discard pile. But for n=35, you'll actually have almost 43% of your deck in the discard pile.
So let's say that, in the average case, Miser will be able to retrieve 40% of your Silvers. That means that to find 4 Silvers on average, you need to have 10 in your deck. (This is consistent with the fact that you start out with 7 Coppers, and Counting House seems to average pulling about 3.) Ten Silvers is a lot of Silvers. In the absence of Silver-gainers, it's rare that you'll ever get this many.
However, do you need to find 4 Silvers for the card to be worth having and playing? Certainly if the Silvers went to your hand, you would be happy just finding two! Then the card effectively amounts to a +$4 vanilla bonus. If you only find one, well, a terminal +$2 isn't very good for a $5 card, but it's not a total bust.
However, the cards don't go to your hand -- they go to the top of your deck. That's a lot weaker, and not only because you don't realize the extra money until the next turn. It's weaker because it slows down the cycling of newly purchased cards into your hand. It's also weaker because in neither turn -- not in the current one, or in the next one -- do you get those Silvers as "extra" cards. Next turn, you get the Silvers instead of an equal number of other cards.
So the monetary increase you can expect next hand must be compared to your deck's average card value. At the beginning of the game, that's $0.7. Except under rare circumstances, that will very quickly rise to over $1. In Colony games, you kind of need to get above $2, but I think it's safe to say this card will be better in Province games, and so we can restrict our analysis to that. To buy a Province, you need an average card value of $1.6.
What I'm leading up to is that your next turn's monetary increase is the total value of your Silvers minus the monetary value of the cards you would have drawn instead. So early on, maybe that's an extra +$1 per Silver. Later, maybe closer to +$0.5 per Silver. Thus, drawing four Silvers means +$4 next turn in the early-mid-game, and +$2 next turn in the late-mid-game.
Well, obviously +$2 next turn is awful, but +$4 next turn sounds pretty good. The problem is, that's when you find four Silvers, and as I mentioned earlier, you probably aren't going to find four Silvers unless you've got ten total in your deck. If you ever get that many in the first place, it'll be the late-mid-game by the time you do, and then they aren't really worth +$4 for you anymore!
So I think ultimately this is not a very desirable card, especially with a $5 price tag. The situations where you might want it are probably:
(1) When junking attacks are in play, fattening your deck and spreading out your good cards. Miser will help you group Silvers together for a good turn. Plus, if you're getting Cursed, Miser will help slow down the infusion of those Curses into your hand.
(2) When Silver-gainers are in play, inflating the number of Silvers you have in your deck. This goes especially for Trader, but Jack and Bureaucrat probably apply as well.
Note, though, that if you have (2) in the absence of (1), Miser is probably still a bad card. Because if you're getting lots of Silvers, that increases your average card value, which in turn diminishes the effective monetary increase you're liable to see next turn. In other words, if you can accrue a density of Silver anyway, there's no need for a card to group them together.
Now, a card that retrieved Silver AND Gold from the discard might be another matter. Still swingy and still subject to a lot of the same weaknesses I've outlined above, but the extra power might be enough to overcome those weaknesses in some decks.
