Generation 5 are some of my favorite games in the series, and the only things that really keep them from being my absolute favorites are the quality of life changes the series made afterward that I very much appreciated, particularly the increased ease of breeding (especially IV breeding with Destiny Knot) in Generation 6 and how fast and easy it is to EV Train in both Generations 6 and 7.
I would definitely say they're worth getting into, though everyone's tastes vary so obviously no one is going to like everything and it might turn out it's not for you.
Best
Escavalier: I like the knight aesthetic in general, so I really like Escavalier and it's one of the standouts for me not only from Generation 5 but across all generations as one of my favorite Pok?mon. It's pretty great with an Assault Vest (though, that wasn't introduced until the next generation) too, provided you're accounting for its typing when in battle.
Serperior: One of my favorite starters in the series, especially since Grass is my favorite type. Unfortunately it didn't get the Contrary Hidden Ability until after this generation had passed, but it was my favorite one to use even despite that.
Cofagrigus: Unfortunate name aside, I really like Cofagrigus and its Mummy ability. The design is nice and the ability has some utility in certain situations.
Galvantula: I liked the introduction of a spideresque Pok?mon and the name and typing are pretty neat. Fast, frail, fun.
Eelektross: Great design, no weaknesses, what's not to love?
Beheeyem: I like the alien design for Elgyem and Beheeyem and the rewriting memories thing is pretty neat. They're really slow, but hey, that can work on a Trick Room team.
Garbodor: I was indifferent to it at first, but then I came across a shiny Trubbish and grew to appreciate it. Now I'm fond of it. Not the best Pok?mon competitively but I like it.
Swanna: It's a very plain swan, but I like birds in real life so that's fine by me. It was great in the rain with its perfect accuracy Hurricane and Hydration ability. Do keep in mind that Generation 5's weather could be permanent if you had something with Drizzle or Drought like Politoed or Ninetales respectively, so this was a great time for Swanna.
Sawsbuck: The seasonal appearance change mechanic is nice and, as I already mentioned I'm fond of Grass-types, I have a soft spot for it. I especially like the Autumn form.
Excadrill: Not much to say, just really like it. XRileyx touched on the fact that he was great in Generation 5 especially though, so yeah, worth using.
Archeops &
Carracosta: Great fossil Pok?mon. Only unfortunate thing is that Archeops doesn't get a Hidden Ability to replace the unfortunate Defeatist.
Scolipede: It's a great design and fun to use.
Emolga: I'm not one for the Pika-clones, but of them I'd say Emolga is the best and most interesting that we've gotten.
Chandelure: I'm fond of Ghost-types and the design and typing drew me in as soon as I saw Litwick for the first time. Good special attacker.
Haxorus: The axe-inspired design is really nice and it's one of my favorite Dragon-types in general.
Cryogonal: It's just a big angry snowflake. Most people probably don't like that, but I've just had a fondness for it since I saw it. I like Winter and snow, so hey, works for me.
Hydreigon: It might not be gold, but I can't help but think of King Ghidorah when I look at it and that's a good thing. It was a monster in Generation 5 since Fairy hadn't yet been introduced, the naming scheme for its evolutionary line is fun, and it's one of the most memorable of the generation.
Volcarona: My only complaint is how long it takes to evolve. Great Pok?mon.
Kyurem: I like standard Kyurem's design since it's supposed to be the husk of what was once a great dragon before the other two split off from it. I wish we got its return to glory instead of getting Kyurem-W and Kyurem-B, both of which look pretty hideous.
Genesect: Best mythical of the generation, as Nox said. Great design and the Drives are a fun gimmick.
There are others I quite like as well, but this is long enough as is.
Worst
Meloetta: I really like Meloetta's design and concept, but they really flubbed the landing. Whereas other Pok?mon like Keldeo and the Weather trio can change their forms by using an item or other outside influence, Meloetta's form change is determined by the use of Relic Song changing her form in battle and it wears off after the battle is over. That really sucks for choosing a nature, making a moveset, etc. because one of those moves will always end up being Relic Song, a special move, if you want to use Pirouette Forme, a physical attacker.
Kyurem-White and
Kyurem-Black: They're fine to use in battle I guess, but as I mentioned when previously talking about standard Kyurem in my favorites list, it's a shame that they didn't give us the original dragon. They're fine, if a bit busily designed and kind of ugly, but they feel like a missed opportunity for me.
Stunfisk: Just a pretty unfortunate design. I don't hate any Pok?mon, and it has its own aspects that make it sort of interesting from a design inspiration perspective, but...eh.
Conkeldurr: Like I said above, I don't hate any Pok?mon, but through no fault of its own Conkeldurr is just a better Machamp in all regards. Which is kind of a bother to me because I absolutely adore Machamp. So Conkeldurr steals its spotlight a bit.
Musharna: Forgettable, though at least it gave us the Dream World while that was still a thing.
Additionally, Mew is the superior fetus Pok?mon.
