I've actually done a lot of experimenting with this one, as it's probably my favorite class to play. Tried a lot of stuff but I've found my favorite build to be as follows:
6/6/3/5/6
Reave--Radius, Protection, Barriers/Armor
Pull--Area, Expose, Duration & Combo
Drell Assassin--Power Damage & Capacity, Power Damage
Fitness--Health, Shield Delay, Health & Movement Speed
Don't get me wrong, I LOVE the cluster grenades and I use them a lot but I find that the basic 3 ranks are honestly sufficient enough to do the job. I also tend to use weapons a lot as well, so I supplement everything together. Weapons and Pull/Reave detonations for the small fry, and Reave/Cluster for the big guys. Sure, you lose out on some damage from the grenades but paired with a good gun it's actually pretty viable to go with a Reave/Cluster/Gun tactic. I definitely like having the extra health for a little wiggle room and the insane movement speed is what makes the drell so awesome (and reave, but that's not exclusive anymore). I also use Pull as a utility for pulling lighter foes out of cover for either gunfire or another adept's detonation. I'm almost always at the top of the scoreboard, and while the extra damage is always nice, I haven't found myself really missing the Cluster damage all that much.
I've actually tried a lot of weapons on the drell as well, but my favorites are the Indra and the Talon. I like the Carnifex as well, and if you're going with a single-gun build then I'd probably say it's better for its long-range capabilities, but I really prefer to just take the Indra and the Talon. I've found the Indra to actually be pretty good at long-range crowd control, and I use the Talon for most of my mid- to short-range encounters. The Talon's weight is a shred more than the Carnifex, so adding the Indra as well does put me at a bit less recharge than I'd like, but even still I've found it not a big deal (I've got a Talon IX and Indra IV currently, and recharge is about 126%). My biggest thrill of the Talon is that if you combo it with Reave, the heavy hitters go down rather easily. When you're taking down a Brute with 4-5 shots with it, it's easy to get over the Cluster Grenades. Obviously you'd still want 'em for Banshees, and for an Atlas or Geth Prime too.
But that's what I've settled into, and like I said I've tried a lot of different builds/weapons with the drell. Then again, personal playstyle makes all the difference. Best way to find what works for you is to just try it all out and see what you like.