In response to the original question about whether these conversion cylinders are worth getting. Yes, I think they are, but because of the cost they should be viewed as though you are purchasing a second gun.
I have a Pietta 1858 that I purchased from Cabela's last year. It's a great gun, well made, simple and fun to shoot with cap-and-ball. However, since I often stop at the range on my way to work in the morning, I purchased a Howell 5-shot conversion cylinder as well. I felt uncomfortable leaving the gun dirty in the car for the entire day with black powder residue and figured I would have less corrosion problems if I shot smokeless. I already had another .45, an 1875 from Uberti, and was reloading low pressure rounds for it and felt comfortable using them in the 1858. I was not prepared for how much I would enjoy using the conversion cylinder though.
At $250 the conversion cylinder isn't cheap, but the quality is simply excellent. Because it's a 5-shot, the cylinder walls are nice and thick and I find the safety notches to be better than on the C&B cylinder from Pietta. The chambers are also machined to tighter tolerances than my Uberti, which I have always found to be somewhat sloppy (although within tolerance) and prone to seal poorly with cowboy loads. I shoot 5-round cylinders-loads by default, so the loss of one round doesn't bother me. I like to score my shooting and using 5-round groups works well for me. I find myself shooting this gun, with the conversion cylinder, more than any other. I also carry this gun around more with me, simply because I like it and am comfortable with it. It's big and heavy, even with the 5 1/2" "Sheriff's" barrel, but the size works with a backpack or glovebox and I've even stuffed it into the front pocket of a loose pair of jeans with the butt sticking up over my waistband and covered by a shirt. Did I mention it's heavy though?
Regarding ammo, as I mentioned above I reload most of the time. I find reloading .45 Colt to be really enjoyable and have found some loads that I like. For the most part, I prefer using 200-grain lead, round nose, flat-point bullets in the 1858, as that keeps the pressure down on the loads. I've shot 250 grain reloads through it as well and they work fine, but shoot about a foot high and I'm not ready yet to dovetail a new front sight on the gun. The 200 grain loads print about 4 inches high of center, which works for me right now. Note that I'm not giving load data here because I think people are complete retards when it comes to .45 Colt loads and use FUD to scare people off. I use Trailboss and like it, your mileage may vary. The hottest, most unpleasant rounds I have ever shot were cartridges loaded with 27 grains of Pyrodex behind 200 grain lead bullets and I hated it. I've shot Winchester, Fiocchi and Hornady Cowboy ammo through this conversion cylinder without a problem and wouldn't hesitate to shoot them again.
So yes, I think the conversion cylinder is a great purchase and I would recommend it.