Just finished it myself, and enjoyed it quite a bit. There was much more emphasis on dialogue and 'exploration' (not vehicular, but it was exploration of sorts) than I expected, and the main action sequences weren't too long. Its not like Arrival where it was mostly combat focused. There wasn't quite as much gameplay variety as, say, LotSB (boss fights, flying car chase) but there was still quite a lot of variety, more than in most ME3 missions.
Unlike Arrival, squadmates all have dialogue. EDI in particular has quite a lot, and both Shepard and James get some humourous lines (along with a new and pretty amusing item for Shepard's cabin). Squadmates also each get a line of dialogue between the various missions that make up Leviathan. Its not much, but this is only a DLC and even so squaddies get more dialogue than they have in any other ME DLC - I don't think squaddies had any new lines whatsoever in any of ME2's DLCs, so this is a major plus imo.
I haven't yet re-done the ending to see what new dialogue there is, but having seen the script snippets that were leaked a while ago it doesn't seem like there's too much thats new. As a self-contained DLC it was good. Not quite LotSB, but I'd rate it alongside Overlord and Kasumi, both of which I thoroughly enjoyed. Just under £10 for ~3 hours of gameplay, with some replay value in that you'll have to play it twice with the same character (i.e. play it once then play it again taking that same character through NG+) to unlock the top levels of the new weapon mods, and to see different dialogue options, but you don't really make any choices so replay value is limited.
Still, well worth it imo. If this is the quality of all of ME3's forthcoming DLC then I'm very much looking forward to whatever Bioware brings out next. I don't really care if the impact on the endings is negligible as I have my own headcanon for the ending anyway, but as a self-contained story it was very enjoyable.
Story: 8
An intruiging main storyline, with some vibes of classic horror sci-fi at points. Some threads from earlier games get expanded on here. Plus, squaddies get dialogue.
Gameplay: 7
More variety than many of ME3's missions. A range of locations. Quite a strong focus on exploration and investigating in the style seen in the opening of LotSB. However, I didn't find the levels challenging at all. That was partly due to playing on normal with a level 60 character, but also partly because there weren't many prolonged action sequences and other than a pair of banshees at one point (at which point you have access to a heavy weapon anyway if you look around) there weren't any particularly challenging enemies. Unless you find brutes challenging.
Replayability: 6
You don't make any choices, so other than collecting the level 5 versions of the new mods there isn't much point replaying it except to get the ~500 or so war assets for other Shepards
Enjoyability: 7
Value for money: Yes
If you enjoyed DLCs like Overlord and Kasumi, and still enjoy playing ME3 despite knowing the endings, then absoluteley. If you really dislike playing ME3 because of the endings, then this does nothing to change that though. So overall, if you still like ME3 then yes.