Back in the days of Photoshop 5.5, 7, CS & CS2, PS's resizer left a few things to be desired. We also had Canon 1D Mk II & IIN cameras that would yield 8MB file sizes. Good enough specs for 8x10's, 11x14's but stretching it a bit for 16x20's & up.
At that point we purchased PhotoZoom Professional to get better looking wall portraits & used it on anything that was 16x20 & bigger.
It worked fine for us for years. Set the parameters, hit resize & let it run in the background while you worked on something else.
Now we have Mk IV's & 16MB files & the PS resizer is much better than it used to be. We still use PZP occasionally for 24x30's & up but we also use the built in version in PS. It's paid for itself but I haven't upgraded to the newest version as I see no issues with the one we're using.
Long story longer, can you get a return on the investment and do you really need it for the sizes you are enlarging to. Address those questions & you've got the answer.