They're worth it, but for different reasons.
First, this ain't Fallout 3, so it's got a very different style. To me, Fallout 3's DLC (and the game in general) reminded me of a comic book. Very action-oriented, and very out there. If you liked Fallout 3's style, then Dead Money should be right up your alley. It's got a very dark, Emil Pagliarulo sort of vibe. Not usually my thing, but I enjoyed it.
Honest Hearts is very different, and maybe a little "bland". It's not as exciting (for me, anyway), but it's such a beautiful location and there's a lot of really interesting backstory to discover. However, be warned that being that Dan's a missionary, and Joshua's a total zealot, the religious stuff is really, really heavy-handed.
Not anywhere near as heavy-handed as passing comments on the street while actually living in the areas they're talking about coming from, I can assure you from experience. XD
Dead Money was a lot more "on rails" than anything else I've played in a Fallout game (I never played the FO3 DLC so I can't speak to that). Almost all of the reviews I read about it before playing it were incredibly off-base. As a whole, they boiled down to complaining that you couldn't just run through it with a shotgun, yelling YIPPY-KAY-AYE and looting everything with impunity. If you took the time to actually play the game as it was intended (I mean, you are kind of forced into a position against your will and forced to do certain things or certain doom will be imposed upon you, so you'd better play along, right?) it was a lot of fun, and challenging. The story line was a lot more compelling because it had to be. The area wasn't really that big and there isn't a whole lot you can do. You're trapped in the area and the story until you've completed someone else's demands of you. Coupled with the dreary environment, this helped inflate the sense of urgency and heighten suspense levels. It did also piss off a lot of "the xbox crowd."
Honest Hearts, on the other hand, is a lot more Fallout-y. It's a wide-open environment with plenty to explore. The environment is a slight break from traditional Fallout in that it's not a completely destroyed and bleak desert wasteland - vibrant colors and clean water and even rain. As far as compared to the base game, it is largely more of the same, just in a different area. This is not a bad thing to fans of the franchise - this is DLC, not an expansion or sequel. There is a fairly compelling side/backstory to the area to be discovered which is juuuuuust messed up enough that you actually want to find the rest of it (plus, doing so nets you some pretty sweet gear in the process).
Both content packs lasted me about eight hours each and were generally rewarding in their own, unique ways. I suspect any fan of the series should have a comparable experience if they let it. The only real complaints I've noticed have been, as previously mentioned, people who want to run and gun everything without thinking about what they're running toward and/or gunning down.
.... man that was a lot of words to say "yeah, buy them."