As has been said, the single most important factor when buying a used BMW (not just e53) is how well the previous owner(s) cared for it. If it was well maintained you will spend about $1000/yr in maintenance. If it was poorly maintained you will spend $2000-4000 the first year, and less than $1000/yr after that.

The above are just rough observations/figures from owning 15ish BMW's. Once they are up to date on maintenance/repairs/upgrades they are as reliable as anything else.

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2005 X5 4.4i Build 04/05 Maintenance/Build Log
Nav, Pano, Sport (Purchased 06/14 w/ 109,000 miles) (Sold 8/15 w/121,000 miles)

2006 X5 4.8is Build 11/05 Maintenance/Build Log
Nav, DSP, Pano, Running Boards, OEM Tow Hitch, Cold Weather Pckg (Purchased 08/15 w/ 90,500 miles)

2010 X5 35d Build 02/10
Nav, HiFi, 6 DVD, Sports Pckg, Cold Weather Pckg, HUD, CAS, Running Boards, Leather Dash, PDC, Pano (Purchased 03/17 w/ 136,120 miles)