I've added a few of these over the years; I found them in the used bin at Amoeba or the local record store. The 1/2 speed versions were a duplicate to standard pressings I already had. After a good inspection and cleaning the results are mostly mixed. Steely Dan Aja was worth the $45 used; very clean and vibrant compared to standard recording. Doobie Bros Captain and Me also very clean, but no real sonic difference from the standard issue. Seals and Crofts Hummingbird just a slight improvement from the standard; mostly with the vocal harmonies.
Contrast 1/2 speed versions to BetterRecords.com premium selections of standard pressings. The two I bought from BR; Steely Dan Katy Lied and CSNY 4 Way Street, were very clean and sounded very detailed compared compared to record store versions I had. BR is very pricey, but offers the ability to send it back if you're not pleased.
As for 45rpm LPs; I have a few that I unknowingly bought (nothing on the cover said 45!) these too have a nice detailed sound, but I don't have a standard version to compare them to.