Let's take a look at what he was like as a prospect.
Pat Burrell was the first-overall pick in the 1998 draft. A star slugger at the University of Miami, Burrell hit .432 with 17 homers for the Hurricanes, although he was limited to 113 at-bats by a back injury. He was considered a top slugging prospect but his pure hitting skills were fairly respected as well, and he wasn't expected to need long in the minors. The Phillies moved him from third base (his college position) over to first base in pro ball, partly because he wasn't much of a fielder and partly because they already had Scott Rolen.
Burrell got into 37 pro games for Clearwater in the High-A Florida State League, hitting .303/.416/.530 with 27 walks and 22 strikeouts in 132 at-bats. In the 1998 book, I wrote "the main question for Burrell is his ability to hit for average. Some scouts see him as a .300 hitter, with lots of walks, perhaps similar to Jeff Bagwell. Other observers regard him as a .260-.270 hitter, similar to Eric Karros, productive but not outstanding. My guess is that he won't be as good as Bagwell but will be better than Karros." I gave him a Grade B+ and ranked him the Number 20 hitting prospect in baseball.
Moved up to Double-A Reading for 1999, Burrell hit .333/.438/.631 with 79 walks, 103 strikeouts, and 28 homers in 417 at-bats. He ranked second in the Eastern League in OPS. He got into 10 games for Triple-A Scranton, hitting .152/.263/.242 with one homer, four walks, and eight strikeouts in 33 at-bats. A trial in the Arizona Fall League went well, and he got in some outfield time without looking too terrible. He showed excellent bat speed and good plate discipline, and I felt he could be a complete hitter. I gave him a Grade A entering 2000, ranked as the Number Two hitting prospect in baseball (behind Corey Patterson, who turned out to be a huge bust of course).
Burrell ripped the ball in Triple-A in 2000 (.294/.420/.497 with 32 walks in 40 games) and held his own in 111 games for the Phillies, hitting .260/.359/.463 with 18 homers and 63 walks. The frustrating thing about Burrell was that he never really grew from there. He was certainly quite productive, hitting for power and drawing walks, but he didn't show the Bagwell-esque total hitting skills, and his outfield defense deteriorated.
His best season was at age 25 in 2002, when he hit .282/.376/.544 with 37 homers, 116 RBI, 145 OPS+, and a career-best 4.9 WAR. Overall, Burrell finished at 21.9 WAR with a career line of .253/.361/.472, 116 OPS+, with 292 career homers in 1640 games.
Most Similar Players to Burrell by Sim Score: Jeromy Burnitz, Matt Stairs, Jay Buhner, Greg Vaughn, Danny Tartabull, Roy Sievers, Troy Glaus, Jeff Burroughs, David Justice, and Tim Salmon. This group makes sense: productive sluggers with low batting averages. Burrell's 21.9 WAR puts him in company with Joe Carter (22.2), Steve Kemp (22.1), Tito Francona (21.7), and Jeff Burroughs (21.6).
It is interesting that Jeff Burroughs shows up as a good comp for Burrell, given that Burroughs was also a first-overall pick (1969).
As for the Bagwell/Karros comp, Bagwell hit .297/.408/.540, OPS+149, 83.9 WAR, while Karros hit .268/.325/.454, OPS+107, 20.7 WAR. Obviously Burrell was closer to Karros, if slightly better.