Take Trigun, swap out about 75% of the "cool" factor and 50% of the drama factor, and replace it with ecchi and fanservice (and perhaps a touch more comedy that stems from the ecchi), and in a nutshell you have Grenadier. For viewers who haven't watched Trigun, all this means basically is that it's all but impossible to not notice the fanservice (I mean, it's a key element in how Rushyuna reloads her gun o_o).
This is not to say that any of the other factors I mentioned above are absent from the series. The drama in the storyline is not totally lighthearted, and there are a fair few moments where the characters take a moment from their ecchi routine to kick some butt in style (even if said butt ends up being fanservice later). The only thing that will differ between viewers is whether they'll like the presence of fanservice, or whether they'll find it utterly tasteless and distracting from what's going on in the plot. And it can go both ways. I personally don't mind ecchi (I am a healthy young man), and while I don't think it's particularly necessary at certain points in the story, I'm not the sort that gets distracted from the story taking place by a second or two of melons being flashed. However, not everyone is like me. If you dislike ecchi in your drama, take my advice and try another series first.
Character-wise, the two main leads are pretty strongly developed. Perhaps not as strong as Vash was in Trigun, mind you, but they still have their own motivation, and their own reasons for moving forward, all ecchi aside. Complementing the two mains is a somewhat hefty cast of side characters, some which play a major role in supporting the two, and others that will make you wonder why the makers bothered introducing them at all. At the least, the screentime of such characters is kept to a minimum.
In the end, I'd suppose the series was enjoyable. At the same time, I feel like a lot of anime fans out there simply get turned off by the presence of ecchi (no pun intended), and while it's totally understandable, it's also unfortunate, as there are quite a few series out there that hide good storylines under their exteriors of fanservice shots. If you're not a hater of ecchi, I'd recommend giving this one a chance.